Once again, this is for indoor water parks with limited vertical space. Yes, the facilities needed to build the starter tower are quite affordable. However, It gets very expensive when you have to modify roof structures to accommodate new slides. It gets very expensive very quickly, it would not take much for this design to be considered. This design requires minimal vertical space, and you can adapt the destination slides to fit into any where you needed to.
The mechanics for this ride are quite simple, using off the shelf parts and materials. The most expensive part of the design is the fiberglass and steel support structure for the base. At about $100,000 its quite reasonable. You should get some quotes from ride manufactures to see how much a 300 foot slide is these days.