Here is a calculator I made using Excel. Plug in your numbers into the blue fields and it will spit out a monthly cost of flying in the red field.
It assumes you will do an oil change 4 times a year. Also it calculates a month to be 4.3333 weeks (52 weeks divided by 12 months) I realize in the winter many will fly much less so you can do a summer/spring/fall calculation and a separate one for a winter scenario. It does not include stuff like an engine reserve or resell value.
You can play around with numbers such as . .. what if I only fly 1.5 hours a week instead of 2 hours a week. Or what if I tie down for $35 a month instead of hangar for $150 a month. Or what if gas goes from $3.86 per gallon to $6.97 per gallon ? etc. etc. . ..
