I'm partial to corns for their ease of care. They would fit into your budget if you aren't looking for anything super fancy. Plus an adult corn can stay on mice so you wont need to buy rats.

Another one that is easy to care for is the Papuan carpet python. The Papuan carpet doesn't have humidly needs like a ball, or many other snakes would. They are one of the smallest carpet python species and have a great feeding response.