Why is one way twice the price as round trip? Should I wait to book? Any tricks?

My experience is that the European carriers penalize for one way flights. U.S. carriers do not, AFAIK and according to one person familiar with the industry whom I consulted in my situation. I purchased a round trip flight on Condor and threw away the second portion of the ticket on one occasion. I found that if I bought a multi-city they were more reasonable, i,e., in your case LAX to CDG then FrA to LAX booked as one reservation might be the key.

Yes, book your flights multicity if in and out of different cities. You could book another one way on Norwegian. They fly out of Berlin.

In general, only the discount airlines (e.g., Norwegian, Icelandair, etc) offer one way tickets at economical prices for transatlantic flights.

While it is probably too late now, you should have booked a multi-city ticket (”open jaw”) rather than two one ways.

You might find it cheaper to take the train back to Paris, stay overnight, and fly home on Norwegian.

Or get to another Norwegian departure city in Europe that flies to LAX ( or someplace you can get a Southwest flight home.)

TAP Portugal also sells one way tickets inexpensively. My son flew to Madrid on TAP Portugal and back from OSlo on Norweigian Air.

As noted above, some airlines do one -way pricing. If you have already booked the outbound trip,with Norwegisn, you are stuck with the carriers that also price one-way.

Try Condor. Frankfurt is their home airport. They do not offer direct flights to LAX,or SAN, but flights with one stop to SAN are around $800 in June. Or you can fly direct to Seattle or Portland for less than that, and maybe have airline miles to get home to SAN from there?. .

I suggest that you try www.skyscanner.net and put in for suitable flights. (I find it best to click ‘monthly view’ as this shows which days have lowest demand and hence lowest prices).

I think you should also mug-up on the trains in Europe:>https://www.seat61.com

Round trip is cheaper because the airlines want to draw a certain number of price sensitive leisure flyers to fill out given flights. The fine print on the cheap round trip ticket will typically state that the fare type is good for a trip of at least X but no more than Y days and part of the trip will include a Saturday night stayover in your destination.

One very common scenario in rough terms- if you don't have something like a Saturday night stayover on the ticket, you're assumed to be a business flyer on Other People's Money and your company is not as price sensitive as the family of four from Chicago using their own savings to go on vacation. And the airline revenue management department is very good at filing a plane with 30+ different prices and types of fares in an economy cabin in order to maximize profits.

Booking a round trip ticket and throwing away the return leg is probably going to be the best option in the end. The airline don't like people doing this but if it's only once or twice, it's not a big deal. (Habitual 'offenders' can lose their frequent flyer accounts if the airline decides to fire them as a customer for that.)

For the following trip to Europe, and there will be additional trips, what you want is an 'open jaw' or multicity ticket that is a single ticket that sends you, say, into Paris and home from Rome and typically prices out similarly as a conventional round trip. The multicity option is typically buried in the 'advanced search options' when you look to book a flight.

Just do the same thing as you did for the first leg. Find a budget airline that will fly the second leg cheaply, except out of Frankfurt. There's really no other choice at this point unless your Norwegian fare is fully refundable. Be flexible on the "non-stop" issue.

Book it on Icelandair.

One way vs roundtrip on some arilines is crazy as you say. If I can book one way with miles, then I wil use a 'legacy' airline one way, but then for the other way, I use Icelandair. I've been happy with them. This next trip It will be Icelandair both ways. For me better start /finish times, shorter flight times, even with the change of planes in Reykjavik

If you are going to throw away a tix, maybe the Norwegian one would be the cheapest to dump. You can ask for a refund of the taxes paid, at least. That could be more than the price of the actual ticket with a discount airline. Or, wait to see if Norwegian changes your itinerary. If they change it, that can get you the right to a full refund.

So now, book your itinerary open jaw, or multiple destinations. Book direct with the airline, not with an online ticket seller that patches together multiple legs on different airline systems (Expedia, Cheapo, etc.)

Delta has cheap one-way prices from Reykjavik to JFK and ATL if that would help...FYI ...about the only cheap o/w ticket I’ve found with them to/from Europe.

You might want to check which airports are serviced by Norwegian Air Shuttle and Icelandair. Then check fares if Delta goes there. We have been seeing the legacy airlines out of the same airports dropping airfares--trying to put a nail in the coffin of the budget airlines.
Next time, check airfares both directions before booking your trip. You might come out ahead taking a budget European airline into one of the following airports and taking Norwegian Air back to LAX from: Barcelona, Copenhagen, London–Gatwick, Madrid, Oslo–Gardermoen, Paris–Charles de Gaulle, Rome–Fiumicino, Stockholm–Arlanda

You've already been told the "secret" - next time, look for "multi city" or "multiple destinations" instead of "one way" or "round trip" fares. What I want to add is, these aren't just for international travel, but work with any combination of segments. For instance, a few years ago I flew Newark to San Francisco, San Francisco to Calgary, and Calgary back to Newark, all on one ticket, for a very reasonable price; certainly cheaper than it would have been to book three one way flights on those routes.

Frankfurt to Paris is only four hours on the train, and booked in advance, that train isn't expensive. You'd need to come in the night before to make sure you didn't miss your flight home, but even with a cheap hotel, the Norwegian flight back from Paris may be cheaper enough to be worth it.

Before you do any more booking, of anything, come here and ask first; that way, we can prevent you from making "mistakes" and guide you to the best options. For instance, there are all kinds of tricks to booking trains; it's not hard, but there is a learning curve if you've never done it before.

Thank you all so very much for your feedback! I'm looking at United right now, Frankfurt to San Diego, with 1 stop in Los Angeles. If I understand correctly, once we get to LAX, we will have to go through customs, correct? Our flight from LAX to SAN is in just under 2 hours. Is that enough time to get through customs with a carry-on, and change terminals?

Is it time to call a travel agent? I haven't used one since 1994.

Nope! :-)

Thank you all so very much for your feedback! I'm looking at United
right now, Frankfurt to San Diego, with 1 stop in Los Angeles. If I
understand correctly, once we get to LAX, we will have to go through
customs, correct? Our flight from LAX to SAN is in just under 2 hours.
Is that enough time to get through customs with a carry-on, and change
terminals?

Customs is a walk through unless you have purchases to declare; don't worry about it. Passport control will be at LAX.

If you have Global Entry and are sitting at the front of your plane, 2 hours is STILL too tight as you will have to go through security at LAX after passport control, retrieve your luggage at baggage claim, and recheck your bags to SAN assuming also that your inbound into LAX is on time. Give yourself at least 3 hours. Make sure your itinerary from FRA to SAN is on one ticket and not separate flights. If you miss your SAN flight, you will be put on the next flight that has open seats. If you have separate tickets, you'll have to buy a new ticket from LAX to SAN.

The airlines have what they call minimum legal connection times, which let you book a connecting ticket based on having reasonably good odds of making a connection under normal circumstances. These are often different from what travelers call minimum sensible connecting times.

The question is what happens if you miss a connection? (I mean other than the airline rebooks you onto another available flight as required by its contract of carriage) I'll book many domestic tickets connecting through Atlanta with a less than 45 minute connecting time because if something goes wrong, there will be generally another 5-6 flights that will get me to my destination that day and at least a couple of them will have space so I can be rebooked on one of those flights. However, if the first flight does NOT have many or any options to get me to my final destination on that scheduled day, I get a lot less cavalier. I've currently got a 4+ hour connection planned for next summer because Delta has one non-stop flight from Atlanta to Zurich every day and summer thunderstorm season wrecks havoc on southeastern air travel schedules.

I am seeing one way fares from Frankfurt to LAX for around $1,000 on Aer Lingus. We like that airline because you can go through US customs and immigration in Dublin on the way home.