This boat is worth what you can get someone to pay for it. Regarding the price, this is not a Hinkley, it's a Hunter. Here's the ad;
1981 Hunter 33 Sloop-20 HP Yanmar diesel, good sails, roller furling, $3900, no major problems, call xxx-xxx-xxxx
No major problems tells me that it floats (upright), it sails, and that the engine runs. There are likely a bunch of minor problems, like an electrical system that may be a fire hazard, polluted freshwater system, alcohol stove that doesn't work, small leak in the fuel system, old batteries, tired sails, and smelly plumbing to the holding tank. The entire boat probably also needs a thorough cleaning.
What's it worth? The answer is; what you want to pay for it.
Realize, however, that the purchase price is like the purchase price of a razor... It's the cost of ongoing use will far outweigh the initial price.