Lots of choices and a lot depends on if you want to buy a pre-built model or build one.

I've just got this one up and running, still fine tuning it and have yet to get the water bottles for it but it's humming along at close to proper temps and within 2 degrees top to bottom.
The water bottles will help stabilize it once they are in place.

True cooler off of craigslist for ~ $200
heat tape ~ $25
thermostat ~ $140.00
tubs ~ $2.50 X up to 18 tubs
Bathroom ceiling fan ~ $18

For under $450 I can incubate up to 24 tubs