Where I live (UK) I have had difficulty in finding a place to buy a reasonably priced incubator. So I've made my own for about £30. Also made a pretty nifty Egg box.

INCUBATOR
Equipment needed.
Heat mat 12"x12" (£10)
Thermostat. (£20)
I use a switch thermostat that turns the heat mat on and off.)
Large polystyrene box. FREE!!
I found that tropical fish stores (Of where there seem to be loads in the UK) have their fish delivered in these. They are roughly 18" square and have fitted lids.

1) Mounted the heat mat on a piece of think cardboard that I'd covered in foil to keep some of the heat off the bottom of the polystyrene box.

2) Create a shelf or Shelves inside. I did this using 4 pieces of dowel rods (5mm dia) just long enough to penetrate the walls on either side of the box. Putting them cross ways inside the box, allows you to create 2 shelves by laying strong cardboard across
and taping them in place.

3) Place the sensor next to your egg box plug the heat mat into the thermostat and close it all up and your done.

Took me 15 minutes to assemble.

EGG BOX
I use the below Thermometers and Hydrometers. By cutting a 2 holes in the top of a tupper ware box and gluing these over them. (I use a non-toxic glue and I make sure they are air tight so no moisture can escape.) You now are able to read the exact temp and humidity levels inside the egg boxes.

I use albey's setup for the contents of my egg box and the humidity level is about 80% using these measurements.
http://www.albeysreptiles.com/incubate-eggs.htm

Few notes.
The past week here in the UK the temperature has been going up and down as usual. 18c on Sunday and up to 27c today. I've been checking the temp of the boxes and they have been VERY steady the internal temp of the egg boxes hardly fluctuated by more than 1-2f. Even with the difference in temperatures.

I also find by using the upper and lower shelf I can incubate them at slightly diffrent temperatures the high shelf is warmer than the lower.

My thermostat is set to 86f and the internal temperature of my egg boxes are a constant 82-84f.

I would advise that you take a few days out to make sure you have the temps set correctly.

Hope this helps someone.

Jil.