Here is what I dug up on the costs:
Stock YZ250: $6299
Stock Superjet: $6500
Stock SXR: $6500
Pro MX Bike $10,000-$20000
Pro Freestyle
or Freeride Ski $10,000-$20000
BMX Freestyle Bike: You can get a full blown badass one ready to go for about $500-$700.
The Motocross Bikes require wayyyyy more maintenance, however.
Skis have no: Tires, Breaks, Transmission, Clutch, Wheel Bearings, Bushings etc... (These parts wear out all the time on bikes. Even worse on 4 wheelers) I would think MX would be a little more expensive than Jetskiing.