The best value out there is the Accurite digital thermometer at Walmart for about $13. You can measure the temps on the cool side and in the warm side hide at the same time, plus it will measure the humidity.

Anyway, that sounds like it could be your problem if you have no idea what the temps are. Do you have any way of regulating the temps like a rheostat (dimmer) or better yet a thermostat? If not, then it is probably much too hot for your snake.