Don't hesitate to charge a reasonable monthly fee. There's a lot of room to undercut the existing players (newrelic et al) and still make it a sustainable business.
Maybe I will do that, because it will be more convenient for some people. But the Pro version will always be an option for people that don't want to pay monthly charges and are OK with managing their own instances :)
P.S And of course the Open Source version will do the job for most people
P.S And of course the Open Source version will do the job for most people