Apparition is more difficult to master than we probably take for granted IC so I imagine that not the every 17/18-year-old can manage it by the end of their seventh year. Classes only take place bi-monthly at Hogwarts from January through May, and yet there’s an entire  Ministry office dedicated to it, so I imagine the rest of the year (and even during that time frame) is dedicated to helping adults get licensed. I don’t think there’s an established fee, but if it’s anything like a driver’s license there’s probably a $25 fee (I’m too lazy to convert to galleons >>) just for the license while the classes and actual test are free to the public—unless that policy is wildly different in Britain.