Bear in mind there will be huge variations, e.g. hangarage can be had for £100/month "up norff" or £600/month "down in the rich and crowded south east"
It's also very difficult to do a spreadsheet type of thing for unscheduled maintenance because by definition it is unpredictable. The probability varies strongly according the the age of the airframe, but avionics come into it strongly, and past history but that is often difficult or impossible to get a grip on when you are buying it.