the standard old glass ones you find hard to read should be $2-3 at your LFS. granted they take alittle more time to see what your temp. is at but once you pretty much get your heater set you dont really have to look at the thermometer very often- just another option.
im pretty sure it was someone in my reefing club but they posted pictures of the thermom going wild, then pictures of them hitting it with a hammer, then pictures of the pieces, then the caption "i fixed the thermometer," lol
heaters scare me. i would hate to loose a couple grand in livestock over a $20 heater, the only time i had one fail on me was quite a few years ago on a tropical tank. on top of that i was away on vacation. when i came home the fish were fried and in like a white mucus. pretty gross, and even that sucked because it was a nice large school of tetras and tiger barbs (which had fry) and they were all gone. if something like that happened to my saltwater tank.....i just dont know.