I directed this question to my husband. His reply:

It came with the house so I don't really know.
It's cast bronze and semi-custom made so probably a lot to replace the entire wheel.
The biggest problem is wear and would come from sand in the water, causing the soft bronze to erode. So it's vital to filter the intake.
The only problem I've had was it coming loose on the shaft, so I had it apart once to machine the end to accept a QD bushing. These are very handy for attaching gears, belts, or wheels onto shafts.