Purchasing a regional train pass to hike in Jungfrau region?

It all depends on which hikes you are doing, and how often you will use the trains/lifts per day.
I'll send you our favorite hikes in the area, and under "Tips" you'll see the prices of the transportation. (Check your private forum mail; it should say "unread mail" in the upper right of this page.)

I use the Berner Oberland Pass (google it) and I think the break even pt for a 4 day pass is $70 a day. But you should also consider your transportation in and out of the region. Berner Ob. pass is good from Lucerne/Bern on south for the trains. If you, for example, took the lift up to Mannlichen, then down to Grindelwald, and returned by train via Kleine Scheidegg, that would be around $90. Grindelwald to First and back on the lift is $60.

There's a bunch of guesswork involved, unless you have been there before and know your habits.
In any case, don't buy in advance--no advantage to it. If you get a week of straight rain, your plans will change, so buy it on the spot. (I bought my Berner O. pass in Bern, then immediately used it.)

Also, how are you getting there ? A HalfPrice Card can also be applied to your train fare from your first station in Switzerland to the B.O. and your fare to your exit city -- we bought in Lausanne and broke even with Lausanne-Mürren-Lausanne including a trip up the Schilthorn plus a separate trip down to Lauterbrennen for the inside waterfalls. (Paris-Lausanne-Paris bought separately from SNCF.)

We are coming from Paris via train. We haven't purchased any train tickets yet. Since we haven't been to this area before, we aren't sure how much hiking we would be doing. However, after looking at the prices to and from various towns in the region, it looks like the cost could add up.

The cost can add up. That's why, like Shoe, we get a Berber Oberland Regional Pass and then do not have to think about the outlay again and we can ride at will.

The pass kicks in at Bern so you can buy your ticket Paris to Bern, then rely on the BO Reg Pass from there. You can buy your pass (4-day minimum) online. You would date it and show it to the conductor after departure from Bern.

Are you 3 nights in Wengen thus 2 full days plus travel-in and travel-out days?

Sadly, there's no short-cut to selecting the best transportation plan -- figure out your preferred train trips, lifts, buses and then compare 1) full-fare point-to-point; 2) Berner Oberland pass; 3) half-price fares plus the cost of HFC. FYI, Paris to Bern can be reasonable or pricey as well, depending on how late you book it.

Shoe, are the prices you quoted full fare or half fare prices as I am trying to figure out whether to get the swiss travel pass or half fare card. Thanks

The prices I estimated are full fare, but you can get the exact full fare prices here:

https://www.jungfrau.ch/fileadmin/Prospekte_und_Broschueren/Jungfraubahnen_Fahrpreise_Preisliste.pdf

Shoe

Perfect, thank you!