Last year my team used car speaker wire and we plan on doing the same this year. We usually use Street Wires 12-gauge and 14-gauge "Ultra Wire" for our motors and solenoids. It's a great wire to use because the positive and negative are enclosed in a plastic tube making it easier to route the wires. It's also multi-stranded making it very flexible.
We buy the wire from Crutchfield.com for $0.50 to $0.80 per foot.
