If you want to take your snake to the vet for a checkup, wait until his/her first poo, bag it and then schedule a vet appointment. You can't do a fecal float with no feces!
I am not 100% sure on this but I think fecals are well under $50.
I bought a ball python from a show once, my second show-bought snake, to make a long story short it turned out she was really sick and died a couple months later.
If you're buying from a breeder at a show, its wise to find out who they are and how well they care for thier animals. And if you're ever in doubt about the health of an animal you're looking to buy, don't buy it.