I am in the same process as well. I am building one for $20 or less right now. The good quality store bought incubators are $100-$800 depending on the size. I don't know about you, but for me, as a backyard hatching egg business, if I want to make profit I have to keep my costs down.
All you need for a home incubator (and they are easy to make) is a styrofoam (or plastic) cooler, a 25-60 watt bulb (depending on size), thermostat (optional, I opted out), thermometer and hydrometer (Digital is best), and a piece of glass or plastic to view the incubator's insides.
I am choosing a fairly small incubator as I want to hatch out quail, which are very tiny. I am using a 25 watt bulb first, and testing it out before my eggs go in. I want to make sure that the bulb isn't too hot or too cold for my incubator size. If it's too cold I can add a bulb or increase wattage. If too hot i can decrease wattage.
The bulb will be sealed in using a light socket that can be plugged into a wall. The digital hygrometer and thermometer will be at the lowest point, where my eggs will be. I'll place 3 small water bottles around my eggs to prevent hot or cold spots (this isn't necessary if you install a fan). I'll test this out before putting in eggs.
Walmart carries all my necessary supplies for $20 total or less.