I had the same idea when I first started looking for a thermostat. Unfortunatly, it doesn't work. The home thermostats available at hardware and electrical stores are designed to be used with residental HVAC systems only. They are pre-wired to fit in with the wiring scheme of your home, not the standard two wire cord of a heat pad. They also lack the remote probe that is really the heart of an accurate herp thermostat system. The smart thing to do is to bite the bullet and go with the Ranco or Helix systems that have been discussed here before. However, if you are really short on $$$, you could do what I am doing now (while saving for a helix).
I installed a standard dimmer switch to the cord from my heat tape ($3 for the switch, plus an additional $1 for a plastic junction box and face plate to make the whole thing look nice.) Then I bought a nice digital thermometer from Lowe's (the ACURITE hygrometer in-out thermo clock, $18.) This has a 10 foot temp probe that I place right on my heat tape. The top display gives me my ambient room temp, and the middle display gives me the temp of the heat tape (which I can now control with the dimmer.) As an added bonus, the bottom display gives me the relative humidity of the room.
I've been really happy with this system. It's a temporary fix, but one that offers a good deal of safety and peace of mind.
Good luck!