Indicates a price or cost is so high, it stops people from buying or doing something. The prohibitive cost of the software kept small businesses from purchasing it.
Pro Tip 2/3
Not Literal
Sometimes it's used to describe non-financial barriers, like time or skill. The difficulty of the entrance exam is prohibitive for many students.
Pro Tip 3/3
Discouraging
Can imply that people are discouraged from trying because something seems too hard or expensive. The prohibitive level of training required made many reconsider pursuing that career.