If you have power to the motor and it does not run, then yes, the motor is bad. $500 sounds too high for just the motor. Sounds more like the pump and motor assmy. together.
labor would be about right for changing the motor and pump assmy. but again too high for just replacing the motor. Should be able to do so in 1 hour. This does not include diagnostic time.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7