Conveniently, the people who make the hall effect gear tooth sensor, Allegro, also make current sensors. See
http://www.allegromicro.com/sf/0754/ for details. The most a FIRST team should ever need is a ACS754SCB-130-PFF (130 amps max current, and acceptable operating temp range) - those only cost $6.60 a peice.
You could always roll your own but I don't feel it's worth it (the price is low, and you could do more innovative things than current sensors
). That's not to say it isn't worth at least trying to roll your own...