I have been considering the same thing. The phpBB templates allow the administrator to paste some html code for advertising in the top and bottom banners. From what I can tell, most contract with Google or some other web ad agency and they provide the html code that places the ads in each persons browsers. The site owner only gets a fraction of each ad-click.
I'm sure I know enough html to code a static ad but I'm not sure how to put several into rotation. As luck would have it, my step-son has a BA in Computer Science and programs web based services for a living. I'll ask him.
The page count has come back down to 300-400 per day, but I suspect that once football season starts that will change quickly. I could see us racking up several thousand on some Fridays in October & November. I'll worry about that when it happens. If necessary I'll put up a paypal tip jar, and maybe a real one at the Weekend of Champions and Shrine Bowl tailgates. No worries.