This is my $12.00 bator
It maintains a perfect 99.5 degrees, at 55 percent humidity.

I used parts laying around the farm. they are as listed.
1 old tower cooling fan from a discarded computer. (12vdc)
1 discarded cell phone charger. (12vdc)
1 new from store hot water element stat. ($12)(more on this later)
1 old Coleman chest cooler
1 piece of plexi glass
some duct tape.
1 lamp assembly from a lamp I made in shop class 35 years ago
part of an old cigar box.
1/4" X 1/4" mesh wire.
1 40 watt clear light bulb.
1 discarded fry pan with handle removed.

click on the pics for full size

Picture 2 shows the bator done with the hot water stat wired and mounted. This stat did not work the lowest setting would not shut off the light bulb untill 105 degrees.
I ended up using, an old space heater stat I had in the barn, not used for 5 years, its highest settings works perfect for maintaning 99.5
The wire mesh I cut and bent to make a safe shelf for the eggs and chicks as they are hatched, away from the fan and bulb, and up off the floor to allow air flow under the eggs.
The fan runs all the time and is wired directly to the cell phone charger, both 12vdc, just try to match amperage as close as you can, all requirements are labeled on each devise.
the fan is mounted in an end of an old cigar box, and bolted through the drain hole in the bottom of the cooler, (very stable)
I did drill a few 1/2" holes around the side, and a couple in the plexi glass.(all can be covered with duct tape to fine tune humidity).
I tried different size light bulbs, the 40 watt, is what worked for this cooler.

Update: Feb 2014.
I have changed the stat to a Bi-Metal thermostat, It works really great
I also changed the bulb to a 60 watt halogen. Have eggs in it as I write.

I have hatched 5 batches in the bator to this point. Rate has been 99% it holds 12 eggs I had 2 quiters, all other viable eggs hatched to healthy chicks.

Here is my latest project....

I really like the re-purposed mini fridge, the inside is clean white plastic one piece shell, that will be easy to disinfect, the outside shell is thin powder coated steel, with all the fridge parts out it is very light, and super insulated with high density foam, pic one is the original orientation, I will use the back of the fridge as the bottom. and a 40 egg auto turner will fit nice. I used a STC-1000 PID controller for a thermostat, very accurate. can't wait till I set a batch in my new bator. ( P.S. the controller reads C degrees only. 99.5 = 37.5 )

here is the final configuration, using halogen bulbs ( 72 watt)

Setting eggs tomorrow, 2/16/2014, I have it fine tuned to hold %47 humidity, and the PID controller works great at maintaining perfect temp, the nice thing about the PID is that you can set the differential degrees to as low as .3 degrees I have mine set at 37.8 (100 F) which turns off the heat ,( with a .3 degrees differential ) when the temp reaches 37.5 ( 99.5 F. ) it turns the heat back on so temp never varies more than .5 F degrees ever . I will update as I hatch with a hatch rate.

Update! 12/23/2014
Set a new batch of eggs today, set 20. Mainly standard Salmon Favorelle, and bantam Fav's,
I also added a second circulation fan, mainly as a backup if my main one fails. My incubator has been in the barn covered with a tarp since last Easter, and when I brought it in to set up and run, I was pleasantly surprised to find that the PID controller kept its programing, and runs perfectly maintaining temp as described above.

Best Wishes....Merry Christmas:)

Update: Jan. 10th 2015.
13 Eggs made it to Lockdown this morning.

Best Wishes Phil.