Mr Meehan's questions are apropos. Assuming we're talking about 120 v
with a fairly low current then a refrigerator thermostat would work. Head to
the dump and see what they have. If the thermostat won't handle the heater's
current draw then a simple solenoid circuit using the thermostat will work
but may cost more than $20. $20 is a low budget since you will want to put
this in an insulated box to prevent accidents.