Yes that is a very legitimate point, so then make PM available to members with X amount of posts that is a more legitimate reason to give them this access, then to just say you can only have it if you pay.

200 Posts is not someone hanging around on a forum to spam, if so they certainly don't have much of a life to wait till they get to number 200 before they can spam someone.

So lets keep this all within reason, rank on some forums has it's privileges, and 200 posts on this should be privilege enough to keep in contact with others through PM.