Tracfone would be very cheap but also risky. $15 for a phone and sign up for the $5/mo lifeline service.

Risky because tracfone's terms of service seem to promise your number will be lost for just about any reason, and I'll testify that if your service gets messed up, customer service just isn't up to solving much beyond sending you a new sim and most likely causing your phone to be switched to a new number.