My wife and I want to fly from Philadelphia or JFK to Milan but return from Rome but the one way tickets are much more expensive than round trip tickets. How do you avoid the added expense?
What you want is called an open jaw ticket. These days, you find it under "multi-city" or "multiple destinations," on Kayak, Google Flights, airline websites, etc. Sometimes it's hidden (look carefully to find it on the United website, for instance).
These are indeed much cheaper than two one way tickets for most itineraries on most airlines. (I phrase it this way because, inevitably, someone will come on to describe the airlines, like Icelandair, that price all flights one way. For them, booking multi-city is not any different than booking two one ways. But for most carriers, multi-city is the way to go).
As stated before, use open jaw option.
Also, check with kayak.com for bargains.
More expensive might be relative. Don't think of 2 one way tickets but open jaw as stated before. If you buy a round trip ticket you will have to go in a circle or back track to return to your departure city. You will also loose 1 day of sightseeing since you have to be back the day before you fly home. Packing and moving eats up at least 1/2 day. How much is time worth for you? Do you want to do sightseeing or seeing the inside of trains.
NOTE: Search for the trip both ways , Milan to Rome and Rome to Milan. Sometimes you get schedules that fit you better.
"Open Jaw" is a common slang term used by travelers and used to be used by travel agents. But it is not a well known term. It would be better to use the term "multi-city", which is the category I've seen used on every airline website. Harold mentioned it in his instructions for booking.
On the search site, find the “multi city” option. It might be a little hidden but it’s there on most any airline’s website and multi-search sights like kayak.
There will be options to enter 2 or more flights. Just put in Philadelphia to Milan and the date, then Rome to Philadelphia and the date. You will likely need to do a separate search for JFK to those cities and compare which is better.
Booking like this is generally very close in cost to a round trip. So close that the train fare to return to the other city for a round trip would offset any savings.