I've have built cars for myself for under 10k. I did a 70 roadrunner for 4k. That was using a lot of parts I have from parts cars, some trading, and not being concerned if for exapmle the bumpers and other chrome parts were perfect. Also reusing door panels, like if the vinyl was good, gluing on to boards I cut myself and so on. I did the 4k roadrunner in less than 4 months at maybe 20 hours a week for 3 of the 4 months. I didn't do the underside on that one.

For someone elses car, if a lower end resto, I mean, not everything redone or to factory original. Done to be a nice driver, needing panels, I still think your looking at 10k for parts alone on the low end. That would be using the low line of paints like nason or omni. So if your charging 25 an hours, that gets you to your estimate. However I'd be prepared to work some hours for free cause chances are 1000 hours won't cut it on a rusty car.

Now I'm assuming when you say resto, your talking a complete resto, underside and all. Now if your just doing a topside, suspension, brakes, etc... then 35k is actually pretty decent in my eyes to make. I got about a 3rd of that for a car I did last year that was a rotiserrie done car, but didn't need any interior work, engine/trans and the owner did the front suspension himself. Also pretty rust free and straight.