the ZooMed 500R THERMOstat is a decent thermostat for the money, and works reliably in my experience. You can get them online for only a few dollars more than that rheostat you linked.
The ZooMed RHEOstat, however, has had many many people claiming that it cannot dial the temp down low enough for corn snakes, only getting as low as the 90s.
If you want a quick fix, go to WalMart and buy a wired lamp dimmer, in the electrical hardware section, for about $9. Or find the thread on this forum somewhere about how to assemble a rheostat for about $7. I have both and they work fine and will allow you to dial the temp down as low as needed. Of course with any rheostat or lamp dimmer you will need to check and adjust the temperature often.
Don't forget you will still need a good thermometer with a wired probe.