What an OEM might charge for a replacement part often bears no relationship to what that part should actually cost. Small permanent magnet motors are quite inexpensive, unless you get into a custom-designed one that only the OEM can supply.
At any rate, the "cheap" part refers to the speed controller, not so much the motor itself. To vary the speed of an AC induction motor, you need to vary the frequency of the applied voltage. The electronics needed to do this are MUCH more complicated than the simple SCR phase angle control that will produce variable DC for a PM motor.
