So, the first question is if you've played it and really liked it. If not then don't even bother thinking about price until you have. If you play it and it sounds very nice then it's worth pursuing. If not then forget it, it's a 40 year-old clunker.
If you have played it and love then do some research and check out what pianobuyer.com says. The C3 is one on the best-understood pianos on the planet. I don't think the 70s ones have that great a reputation, but that's less important than its condition and what work was done. Is it a two-pedal by any chance? If so then it's probably gray market. If you don't know what that means do some further research. The fact that's it's a gray may mean nothing at this point but caution is advised anytime you're looking at a piano that old. "Recent work has been done" is too vague - the piano may now be worth $3,000 and it may be worth nearly $20,000. It really depends.
Giving a price is impossible without knowing a lot more but it's almost certainly a lot less than the piano store is asking - I've seen stores inflate prices on such pianos by rather a lot. Find a piano technician to go check it out. Larry Fine publishes a Piano Depreciation Schedule that might give you some rough advice but the C series has ballooned in price, so the estimate you'll get from that is probably high.
The trap to avoid is buying a $3,000 piano thinking you'll "fix it up" and then have a piano worth $20,000. That is almost always a losing proposition which is why I said if you play it and don't love it then it's best to move on.