The sticky thread mentioned above would be a great place to start regardless of the cost of the boat. I used it to evaluate my own boat, a bit sobering really. Don't get me wrong she is solid and sound but the list of things to do didn't get any smaller after the exercise was done
With that said, lets assume you buy a 4000$ boat and spend 800$ on a survey. It seems high at first glance, almost 1/4 the cost of purchase price. But, purchase price is just the beginning. My 26 footer costs me about 2800-3000$ a year just to own, never mind any repairs or improvements. Add to the equation that it is a lot easier to buy a boat then it is to sell one, I would say a survey from a competent surveyor is a must.
If you are thinking of purchasing a 3-6000$ boat, good for you, there are plenty of them out there. But get a survey before you lay your money down. Surveyor's have no skin in the game and can give you a more realistic view point of said boat while you are ignoring the faults and dreaming about the possibilities.