Look at the low end (cheap, not low current) speed controllers used for hobby brushed motors. I can pretty much guarantee that they will be cheaper than the price to build one yourself.
An example
http://www.bphobbies.com/view.asp?id=A0780190
I have used this company a few times, they are in new jersey (not just a front there, they actually ship from there)
It may surprise you to see they have a 10A brushed speed controller for $13
Would I trust my $$$$ helicopter to this super cheap speed controller, probably not, but something that will not "crash and burn" if the speed controller dies sure.