My point behind increasing the cost of PrisonPearl is to make it a harder decision than it currently is to keep someone trapped. That is, if you're going to invest X amount of diamond blocks (doesn't need to be diamond or blocks, just using as example) then you obvious have a very good reason to keep them pearled (raiding base, etc). Otherwise there is no point in keeping the player pearled, other than to inconvenience them, which for some players is going to cause them to just leave the server and find somewhere else to play.