Mod motors come from budget to expensive, I've seen new motors as low as 25.00 to as much as 75.00. Of course the higher priced will give you horsepower but sometimes slower is faster in lap times. It depends on your skill level and driving style. We use Orion motors with enduro brushes to tame the motor and get extended runs. My son doesn't like the feel of brushless motors will stick to brushed only.
My son went all brushless last year and we couldn't be happier. He loves the feel of the LRP systems and, except to do routine car/diff maintenance, the motor hasn't been out of his B4 since the nats last Spring!
Machine wound mods are inexpensive as previous posts state. The expensive ones are usually hand wound. Any time a person takes time to manually do what a machine does, the cost goes up. However a machine doesn't know or care if the wind is smooth or rough and straight or crooked. So you are paying for the expertise of the person who wound the motor. I am still running a 19 turn mod in a class that also allows 10.5 brushless. Last night I was 5th in the A Main. Every other was a brushless. What you run is personal preference.