As another alternative, I would like to see these used. They are definitely a cheaper alternative:
http://www.canakit.com/50a-dc-pwm-mo...ontroller.html
Especially if you buy in bulk here (10+):
https://www.sparkfun.com/products/9668?
Also, to whom mentioned a lower amp version, they sell a 30A that is just slightly more that 1/2 the price:
http://www.canakit.com/30a-motor-spe...oller-pwm.html