There's something to be said for "you get what you pay for". I'm going to guess that the under $100 that you've found are hovabators. Hovabators are designed to hatch chicken eggs, not reptile eggs. Certainly people have used them with some success, but I've heard far too many losses to ever feel comfortable putting any clutches of mine into one.

You can probably make your own incubator for under $100, but you will spend over $100 in the end once you add your thermostat, which will run you $80 to $135, depending on if you get a Ranco/Johnson's (on/off thermostats) or a Helix/Herpstat (proportional thermostats). I am using a Helix on my incubator, which is a converted dorm fridge, that was gutted, has a strip of 11" flexwatt taped to the front door, and a 12V fan inside the unit.