Expensive is a relative word so that is almost as hard to answer as asking if a pool is “expensive”. A vanishing edge (where water “falls” over the edge of the pool creating a visual effect of “vanishing” water) is a completely independent hydraulic feature that requires a separate pool structure (the catch basin) and plumbing system (another pump and filter which is sometimes a “large” pump). There is a fair amount of material and labor required to create the system and of course, it requires a level of expertise that is not easy to master. So in terms of water “features”, yes it is an expensive one. But depending upon the budget of the client, it is sometimes an integral part of the project.