What would you do if the forum had a subscription fee (to post classifieds, etc)?

tsk, I think we moved from a lower tech version of the board, because of the take up in members, it wouldn't have survived with the numbers that were using it, and vBulletin allows our illustrious leader more space and time to turn off and on functionality when required. I'm willing to be corrected on this if I'm wrong.
Not sure turning off functions would make the forum cheaper to run though..

It's true.

The features of vBulletin allowed us to grow. Vanilla did not support caching posts, using RAM for frequently queried stuff, using a content distribution network, compressing the output to reduce bandwidth, etc. Let alone the management side of things... Vanilla didn't even let me deal with spam easily.

Basically the more advanced and feature rich forum supports a greater number of users at lower cost.