cheapest way...

cooler -- $10
water -- free
4 peices of plastic to hold egg creat out of water -- $1.00
egg create -- $5 for big sheet
vermiculite -- $10 for 10lb bag
plastic shoebox tub -- $3
under water heater -- $20

usually you have an extra tub laying around as well as maybe a cooler and something to keep it elevated out of water. you can find alot of diy cooler incubator videos on youtube.