I subscribe to Woodchuck's Second Law of Vintage Auto Valuation which is as follows:
A = B - (X + Y + Z) where....
A = What the car you are considering is truly worth...
B = What the average price of really, really nice examples of the type of car you're considering have been selling for...
X = What you can buy the car for, including the acquisition costs (taxes, fees and transportation home).
Y = What it will take to put the car into the same condition as those really, really nice examples you saw. Note: Don't forget to value YOUR time and those expenses that are necessary but not usually considered... gas, electricity, heat, tools, supplies (sanding discs, welding wire, etc).
Z = What amount of equity you'd like to have in the finished product. This can be zero or it can be some amount that makes you comfortable. It can also be a negative. If you think it's worth, say, $5,000, to you to have the car you really want, then that's YOUR decision and nobody else's business.
So, a hypothetical example is that if really, really nice '66 V8 hardtops with a 4-speed (3-speed manual is actually a deduct.) are selling, on average in your area, for $16,000 and it's going to take $15,000 to get this car in that shape and he wants $4,000 then you'll be $3,000 in the hole when done. Whether or not that works for you depends on your motivation. If you're looking at the car as a "flip" then you probably want to pass. If you're looking at it 'cause it's what you really want, then it may well be worth the money if you are pretty sure that you have the patience and where-with-all to see it through to the end.