For a system this small, you will see far more cost in the gate drive and balance of system then in the actual transistors.
Say you want to drive a small single phase induction motor, 1 horsepower at 120V. You rectify your supply to get DC at about 170V DC rail, and then you use 4 transistors in a full bridge to synthesize roughly 8A RMS to the motor. You could _borderline_ get away with MOSFETs or IGBTs that have a 20A 200V rating. You could find parts that would get the job done for under $2 (note: you need 4 of these to make the full bridge.)
But in addition to the actual transistors doing the switching, you need to supply gate drive to run the transistors. The gates of large mosfets are capacitors, and you need to charge them quickly. So you have to add a gate drive chip. Then you have to add the fact that the 'high side' switches in the half bridge need to be driven with a floating supply.
If you want to switch at 20 KHz, I'd expect gate drive to swamp the cost of the transistors themselves.
-Jon
