Towers, Come on - 30000 members and 10 million hits a month, income less that a 1000 quit a month - you gotta be kidding us!
However if you're right(of course you are!), I would start charging a lot more for those banners, or even better, I would contact a professional marketing company right away, and let them run this banner business.
The banners are not gonna disappear in the future, unless the members will have to pay for using this page, you gave the answer yourself to this idea - not good!
So why not get the most out these banners right away. With professional help you wouldn't have to think about how to finance the state of art server solution, and further more, if you continue to do this work without any pay as you say you do, you would have plenty of money to sponsor a great deal of PPRUNE Bashes, a great deal of wannabees, on top of the costs for the server. So what are you waiting for?
Best Rgrds
Moonbeam