Well.... Don't think me a fool, but I haven't exactly come up with a number just yet. I'm still trying to figure out just how much to budget for each type of plane.
Someone gave a pretty good rule of thumb to me once, well after I was an aircraft owner, but I shared it with my wife "just in case"...
"If you aren't liquid enough to replace the entire engine tomorrow, you aren't liquid enough to own an aircraft."
(Replace engine with plural if you're flying a twin. Maybe that one's a bit much, maybe not -- the twin drivers can comment.)
It's a good quick-and-dirty way to see if your spouse is REALLY supportive of the whole idea. Engines sometimes die painful immediate deaths, and if you or someone in your group can't crap the cash to get the aircraft back in the air, it's a long painful grounding.
I have TWO friends who acquired entire aircraft this way. Co-owners were in completely over their heads, aircraft sat "too long" (for varying values of "too long") for an engine replacement, negotiations were held, and the "new single owners" walked away with an airplane that they still had to buy an engine for, for their headaches of waiting and wondering if they'd ever fly again.
One was a C-150, the other was a C-182. The C-182 not only needed an engine but also needed bladders immediately thereafter. That owner is one PROUD owner of the C-182 he dumped a LOT of cash into in about two year's time. His co-owners are airplane-less, and were "bought out" by the purchase of the engine.
So as a rule of thumb, but not necessarily a good indication of what's going to happen if you get GOOD pre-buy inspections including a look inside the engine with a borescope, it's pretty accurate.
The question to ask yourself is, "Can I hang a new engine on this thing TOMORROW?" If not, you're probably better off renting.
In our particular setup, a new engine split three ways, I could do. Two ways, harder. By myself... that'd hurt a lot of things I don't want to hurt. Co-ownership can either lessen the pain, or make it worse... as noted above.