Phil, won't just an on/off switch do?
I know they would be on full all the time the switch was switched on but at least it wouldn't cost an arm and a leg. ie you could switch them on till your hands get hot then turn them off, and on again when required.
I have a set of Vicma heated grips fitted on the Suzuki. They were £30 from Wemoto and are excellent. They use the old style controller that Oxford used to use until they went all fancy-pants.