I've been watching these guys for a while and
finally talked to one of them about the sport.

Typically, a good kiteboarder will have five kites
from 12 to 25 sq feet @ about $400 each,
plus a board, and line ~ $350 more.

I';m told it's about $2500 or so, to be set up for most
conditions.

http://www.kiteboarding.com/

I'm impressed that the kites are now inflatable and
it's possible to launch them again, if you goof and
put them in the water.