How much did it cost you?

I do most types of work for customers and quote stuff fairly regularly. You will always spend more than you think. Buy the best car you can afford, and talk about the risks with the wife. It's not different than spending $30K on a minivan of six years, but it's only being spent because you like it and want the enjoyment. So it's definately a "discussion" deal. As for "how much"... I get $500-1000 a side for full quarters depending on what else is bad... and there's always something else. Rust repairs, bodywork, and paint $50/hr. So cost is entirely dependent on the car. You can make it easier by buying a car that has a lot of reproduction body parts made for it. I did a '71 Bee last year and it was very expensive because we had to buy chunks and harvest parts. That bill is $8K just for steel work: one full 1/4, cowl cap, firewall, one inner fender, rear floor pans, trunk extensions, lots of smaller rust repairs, and left the car in etch primer. On a car that there are few parts for you're doing a lot of the work twice. The last car was a 5.0L mustang I did some frame rot repair, painted the engine bay, added some suspension, removed hood pins, and painted the hood. After materials (he provided the suspension) the bill was $2500.
The '68 Barracuda in the shop needs a rear floor and I'm guessing that will end up in the $1500 area depending on how bad the frame rails are.

Edit: I'll add - rust always goes down. If the roof is rusted through - everything below should be suspect. Vinyl top cars were not painted under the top - so they are typically much worse. I'll also note a lot of cars for sale now have old, amature work done and that is coming through already. Be wary of fresh body work, and cars with a of of filler in places that should not have it.