I am flying an old O-300 with about 27l/h Mogas STC and do derive a TCO cost monitoring. At 100h per year I have total costs monitored at about 190 Euros/h, with all buying depreciation, avionics upgrade to GNS430 and SID included.
This is within my budget. If I can fly a nice 4 seat C172 for that money, I'm happy. Unfortunately, most 172's I see for sale at the regular website don't give me the confidence of a good deal. I suspect people who own a good maintained plane are not very willing to part with it without asking a very steep purchase price. I haven't seen any good mid-time 172 with SID done for 35k.