I'm not sure about cost, but consider this:
1) To do the mods, you'd have to have the wings off, which means you'll have to have them remounted afterwards.
2) There is information about the mods on Vans website, I'd download this information to have a better understanding and also to help get quotes.
3) If you are buying a flying 3, which you want to convert to a 3A (mods done), then it would be wise to have it completely looked over by someone who knows RV-3s (pre-buy inspection). This may add cost.
4) All that being said, there are some great-looking 3s out there, selling for cheap that just need some TLC. I know of at least one that's being restored after tornado damage (Tony Boy II), so where there's a will there's a way... to owning Van's "best performer."
Good luck!