That is something that too few students think about. Let's say you are going with an operator that charges $85 for a two tank boat dive. (some charge more; some charge less.) Since it takes two such experiences to be certified, the student is getting $170 worth of diving PLUS an instructor included in the course price. That is money you would have had to spend anyway if you were already certified and just going for the experience.
Just for comparison, a single day lift ticket for skiing in most of the areas around here is $120. For not much more than that, you are getting all your certification materials, all your instruction, and a couple of days of diving. Even the most expensive scuba instruction is downright cheap in comparison to many other activities.