How much would you spend on it?

If its solid with no rot its restorable... 3-5 grand and it would be a cruiser. Fully restored its a 12- 15 thousand dollar car in the real world. Yes people pay it check out Maple Motors and similar car lots. Now I'm not saying you wouldn't spend that in restoring it though but really... if interior can be cleaned and you paint it? how much? say you have a 361 or 383 to toss in it... you do the body and shoot the paint? a single stage ? you'd turn a profit at 12-15 grand...you low baller guys think low ball IMO :lol:
Fully restored will cost a lot more than you will get in return on a C body, unless you do a the work yourself (just noticed you said that). And for a '66, the demand is less than the fuselage body style. 383 will work, 361 would drop the price considerably, the purists would walk away.
I think its changing though yeah 20 -30 years ago c bodies weren't wanted but now?... I work with a guy who occasionally drives his 68 Polara in and its a 383 car a huge boat to me but he loves it. it isn't junk either some of these big cars were taken care of garage kept etc. The mindset that "its a c body part it out..." may want to rethink it especially if the car is savable.
C bodies are still cheap, comparatively speaking, but yes, prices are on the rise. The owners are often a bunch of Cheap Bastards, me included. Sure, we spend money on our cars, and most of us keep them because it's a losing proposition to sell, in most cases.

I buy a whole car to get the few parts I need for my rides, then sell off what I'm not going to use. Do I want to keep every car I come across? You bet, but time, space, and money usually make that impossible. I got my first C body to flip, and raise a little coin for my Dart. Well over fifteen years later, and I've got six C body cars, and two C body parts cars. The poor Dart still sits patiently waiting it's turn. The C body world isn't a huge financial gain environment. But deals are still out there, if you're constantly searching, and have the funds available, they can be had for cheap.

My Slab Side Polara is just bigger than a second Gen Charger, and not much heavier. The Fusie cars got fat.