Spring from McMaster Carr: $4
Spare materials, nuts, bolts, etc.: $0
Shoulder bushings for pivot bolt: ~$3
Spare 101a durometer skateboard wheel from local shop (very important to support local skate shops!!): $6
Cheap way to make a tensioner. I was going to turn a groove in it for the chain, but never even got around to trying it out. Not sure I'm going to need to run it. So far I haven't had any problems because I'm only stretched 4.5"
Serpentine belt tensioner.You'll need a wheel though.You could always run a gear instead of a wheel.I thought this one up for my BSA when places wanted $150-200 for their pre fabbed "one size fits nothing" setups.The skateboard wheels seem to work well.