First of all I don't think this will become a Pingdom clone/killer. Ease of use, simple, and unobtrusive are values that I will continue to value when deciding on where to take the project.

If I do decide to take money for premium features (more monitorings, higher monitoring frequency, history/statistics) I think I will do so with the hope of getting enough subscriptions to cover the server costs. If it provides more revenue than that I could see myself providing SMS notices to non-paying and paying users alike.