If you're determined to spend less than $100, look at Vivarium Electronics. The VE-100 is in your price range, and a better way to go than the HydroFarm. As far as proportional vs on/off, if you're using a natural substrate like cypress or repti-bark, they hold heat well enough to maintain a constant temp even with an on/off Tstat. Just don't use one for "white light" heat lamps. Aside from causing stress, it's annoying as hell having lights turn on and off.

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