Well, if you're looking to spend under $100, you're not going to get a good quality incubator. Unfortunately that's just the way it is. You get what you pay for. Even a Hovabator, which is Styrofoam and on the lower end of what I'd consider good quality, will run you $125 if you get it with the fan, which I would highly recommend you do since still air is harder than forced air to incubate successfully with.
The step up is the Incuview, which is very nice and much easier to use than a Hovabator. That costs $175.
The other option would be to make one yourself, which may save you some money.