On the reptile basics site I think they have a johnson thermostat for $80 that has a good display and is calibrated, but if you want to go cheap just adjust the reptitemp to your thermometers, just put the 2 probes next to each other. As always with a cheaper thermostat get the $10 light dimmer switch from the hardware store in case the t-stat fails.

When these fail they typically fail in the on position and can burn your snake. The more expensive t-stats have a failure setting that defaults to off. As to how realiable this is I can't say, I don't have one yet.