It's cheaper to make one yourself. The better incubators have preset thermostats, a no brianer that eliminates need for tweaking temp throughout incubation. The gas filled wafer thermostats are indistructable, inexpensive and proven. Only problem with them is the metal will relax with use requiring the temp to be nudged down in first week. Unused metal wafer at start takes more heat to expand and less as it's in use so your temp will rise slightly over the first few days to week of use then be rock solid.
Inexpensive models with metal wafers look no further than a Hovabator economy model.
Reasonable preset look no further than Hovabator Genesis.
DIY kit where you make the box and slap in a preset unit with fan included for $50: Add cost $50 for auto turner from local farm supply, wood and plexi from hardware store and that's a kick butt incubator for $120.
http://incubatorwarehouse.com/48-watt-incukit-dc.html
Note that this smaller unit is for desktop model size only, attempting to use it in larger space will result in it burning out. Make your box sized to fit an auto turner and your good. Drill some holes in bottom and top of box for air flow

Easy to use plexi or two layers of plexi spaced for insulation as part of top view lid.
ETA: make the box slightly larger than auto turner so you have space to put cups of water for humidity control. Humidity is control.ed by surface area of water present. I get 35% RH with a double shot glass or tumbler and 65-70% RH for hatching days with two large mouth coffee cups sitting right in incubator.