I had a look at that once, its pretty expensive for a message box to be honest lol. Most forums i go to have a 250 messages box and such. I always wished it would be expended as well, when i was using PMs more it was often full. I tend to like to keep infos and all. Having to screenshot messages just to keep them is a bit weird.
If it was a one time deal i would had probably partook a while ago, or maybe a year payment of 10-20$, i don't remember exactly but it was at least a 20$ a month thing. I guess "Reds" would be better placed to answer this.