$15k. You will easily spend $50k to make that car decent, unless you are a bodyman and have a machine shop AND a paint booth.
Also, I wouldn't count on it being "worth" $80k or some big number like that. You might get someone to pay that, but I'd check the number of Challenger converts on the market, waiting for buyers at under $50k. (I found three last night). For a real R/T to be worth serious B-J money, you won't be putting on any P&G Classic parts, will you? For that money, I'd want all NOS or restored parts, no repros.... and that will be expensive...
Here's a nice Chally rag that has been sitting for more than a year, waiting for a buyer at $85k.
http://www.familyclassiccars.com/ourinventory.html (click around till you find the Chally convert)
Plus, on moparts there is another one (not a real R/T tho), in AZ, all bodywork done and painted, with most of the parts there, and the guy has had it listed for like six months at $35k obo.
It's still there.
There's a nice orange 340 shaker car for $49k out there too, and the flipper that's selling it only paid $44k for it.
So I'd give that one on moparts the ol' walk-on-by...