A good problem to have

I have a web app hosted on google appengine. I built it for folks who suffer from chronic disease (including myself). Anyway the app has grown to about 1100 users who visit every day and now appengine wants a couple of bucks every few days to keep the lights on. Right now the app has no advertisements and is free. I would like to keep the app free with ads or charge a nominal subscription -- say a buck a year. I think that certain companies who cater to my disease might want to advertise on it.... I am not sure how to proceed in the best way.( I got in to engineering not marketing/business after college ) Any suggestions on a good way to monetize this thing?

Put on some(only some) google adsense. They pay really high for any disease keyword.