Most bang-for-the-buck would be 3800 S/C. It will bolt-up to the Fiero transmission. Beyond that, you'll have to build everything else: front motor mount, exhaust, wiring harness etc. There's enough information here to walk you through it if you're mechanically inclined.
Probably the easiest upgrade would be to go to the 2.8 that came in the Fiero. Since you have a 4 cylinder, you would need an ECM, wiring harnesses and all the parts that bolt onto the V6. Grabbing a dropout from someone's engine upgrade or a wrecked Fiero would give you everything you need. If you could get a complete blown 2.8, you could go one step further and drop in a 3.4, and bolt all the Fiero V6 pieces onto it.
The 3800 SC (in phonedawgz's pic) came in a number of GM vehicles and is a very popular choice for swapping into a Fiero just because of the power gains, ease of the swap, and cost of the swap. Converting a 4 banger to a 3800 supercharged isn't much harder than converting it to the stock 2.8 V6.
The Search function really is your best friend. You'll get as many variations of the "best swap" as there are - swaps. You need to do some research here on the Forum and decide what swap fits your skill, ability and wallet. "Ease" of swap is directly dependent upon your ability to do the work or your ability to pay someone to do it for you. Cheap - none are cheap - even swapping in a Stock 2.8 - if you are really doing it correctly - costs a fair amount of money.
My 0.2 cents - the 3800SC is tough to beat for expense and difficulty - the 4.9 Caddy is not difficult and can be done reasonably. Just remeer that anything other than a Stock Fiero for Stock Fiero swap is going to require some custom fab work, custom wiring work and a lot of "extra" odds & ends that add up pretty quickly. Good luck on your swap - you did come to the right place!
"Bang for the buck", I just sold a good running 2.8 with a good trans, on a cradle, with EVERYTHING needed (exhaust, wiring, computer, to just" bolt it in' and "Plug it in" for $200 ! (guys replacing a bad 4 cylinder) Keep looking, post wanted ads, They are out there. Find someone who's removing a 2.8 for a engine swap to a 3800 !