I feel like Jay H above when he said he never added it all up because he REALLY didn't want to know. I am afraid to add it all up I just stick my head in the sand. lol.
Body work, fixing rust, etc, I've spent somewhere north of 4,000 on that, and the paint job will be about 2000 more, so lets say I'm at the point of at least 6000 just to have a rolling painted rust free body. So if you take AnYTHING from that, then understand a rust free project car is worth it's weight in gold practically.
Engine and transmission, I bought my neighbors 85 Cougar for $300 after a fire destroyed the wiring harness in the car. It had a Ford replacement motor with only about 4,000 miles on it, so in my mind I got a basically new 302 engine. I am also using the AOD trans
from the Cougar, but thats original not rebuilt so I might have some problems down the road. I changed the 302 over from throttle body injection to carburetor, and along with the brackets and oil pan, etc, I am under $1000 for engine and trans
Suspension & steering, I didn't reuse anything, but I did find quite a few good deals on ebay. New springs, upper & lower control arms, new bushings, new spring saddles, V8 steering linkage, Granada disk brakes, new calipers, new rotors, rear springs used V8 leaf springs, 1957 Fairlane 9 inch rear. Probably around $1000 so far on the steering and suspension, but I still need to upgrade the rear end to disk brakes and I still need to complete the brake system (new lines, new hoses, new master cylinder). So lets estimate $1500 by the time I'm done.
Interior has not been touch yet, although I did get a full set of seats for $120. I'll need to have those recovered, need to have headliner put in, new door panels, dashpad, carpet, etc. Most of my catalogs have "interior kits" for like $400 so I'll probably go that route. I am hoping that $1000 will take care of the interior and electrical (I have a set of gauges picked out already).
As you can see from above thats already $9500 total, and I have not addressed weatherstripping, chrome trim (windshield, rear window, bumpers, side emblems, hood lettering, taillights, etc), exhaust setup, wheels & tires, or sound system. It's depressing to think of how much the total will be, because this is easily going to go over 12000 but in the end if we had to sell it would probably sell for 7,000 or something like that, which is about it for a nice restomod coupe.
Good luck in your project but if you are doing this to make a profit you're in the wrong business. If you are doing it to have a car you love made the exact way you want it, then definitely jump right in!!