Does it cost more money to do this?
Money no, performance hell yeah.
Think about it. A database has overhead. Everytime you click a page or login it has see if you have unread messages right? New pm's and such.
Ok if everybody lets 250 messages build up it causes overhead on the server. Even if someone quits the site and never comes back those 250 messages are sitting in there from now till forever.
It used to be 150. Now its 250.
If it used to take 100ms to scan my 150 messages it now takes 190ms to see if I have new mail. Thats everytime I log in, everytime I click a link, everytime I do anything on the site.
Multiply that by 100 people online doing stuff at once you get some load.
Multiply that but 500 people online doing stuff at once you get a bunch of load.
Every extra point those message limits are raised you exponentially increase load.
I want to make sure everyone has enough space for that but thats why I dont pick huge numbers and instead just raise it as needed.