My recommendations:
$30 Hydrofarm - Will do just fine for your intended use. Consider upgrading to a proportional at a later time.
$100-$400 SpyderRobotics Herpstat, Helix, RBI VE-200+ - Cost's about $60 more, but with the benefit of proportional adjustment.

Not recommended:
$5-20 rheostat - Might as well shell out a little bit more for a thermostat. Your time saved from not constantly making adjustments is worth far more than the negligible price difference.
$? 500r, Zilla - You get a lot more with the Hydrofarm.
$60 - $85 Johsnon/Ranco - Not any better than a Hydrofarm. At this price point, just spend a little more for a proportional thermostat.
$85 VE-100 - It has nice features, and it's a little more advanced than the Hydrofarm, however it's still non-proportional. Again at this price point, just spend a little more for a proportional thermostat.