It costs $100 to fly with a wakeboard, so anything under that would be ideal.
I've been told (I haven't done it myself, so I don't know firsthand) that if you tell the airline that it's a snowboard, it's free. They do this to encourage travel for ski trips. If you tell them its anything else, they assume surfboard (especially if you say its for water) and they charge you an arm and a leg. I'd try saying it's a snowboard.