If you absolutely have to have a viper (or pit viper), I'll just repeat what others have said. Go with a copperhead. You should be able to find babies for around $10-$15 at some of the venomous shows, adults for $25-$35. You can also get cottonmouths for around the same price, but a bite from one of them is highly necrotic.
But i'll also repeat what I've said. I'd start with a less toxic rear-fanged snake, like a false water cobra, or some of the CB cat snakes. Even a madagascar giant blonde hognose snake would work. Stay away from most of the vinesnakes, as they're lizard specialists (For the most part).