The tower that you are referring to is a Rad-a-Cage Tower not a Roswell tower.
Sam is correct for the tower set up that is on the 08 GG boat with the z5 you will be looking at a minimum of $4K.
Roswell does make towers and they also make accessories for the Rad-a-Cage but if you saw the boat at the boatshow and it was an 08 GG boat the odds are that it is a Rad-a-Cage not a Roswell tower.
http://www.roswellwakeair.com/