If you want to have a good idea of the cost of ownership, assuming you fly it at least semi-regularly, take the cost of the rental for the same plane (same condition, same equipment, same year, etc) and take off about $5. That's close to what it will cost you to own it, make the payments, pay for the maint, fix a few little things that may break, and so on. FBOs aren't raking in $40 of a $140 rental, except in some rare cases of overcharging, which is pretty transparent. When I've ran the numbers, it comes out just slightly cheaper than renting. Fly it less, and the cost per hour goes up, so it's in your best interest to use it. That's where it usually falls very short, you were only planning to put a bit of time on it, and spread it out. That's fine, but it means your cost per hour is likely very high and it's just sitting there doing nothing. Consider this very carefully.