The answer of course, will depend on your resources available and your abilities. It could be anywhere from $300 to $3,000 or more, depending how you approach it.
If you have a local junk yard which you can pull the parts yourself, you could probably squeak by around $300 if you didn't rebuild any of the parts. If you had a shop do the labor and put in ported heads with aftermarket cams, it will certainly be on the other side of $3,00, maybe closer to 4k.
As for power gains, on a mostly stock setup going to stock PI stuff, you should see around 40-55rwhp. With a tune, you'll be on the higher end. Obviously, ported heads and cams will give you more, even up to 100rwhp with the right combo.
I went with just a PI intake and PI cams. That'll give you 80% of the gains with half the work. I have been contemplating throwing some PI heads on, now that I have them in hand. Might do a little light porting and give it some new valve seals to make it worth while.