What model C172 or PA28?
I have a 172M with the O320-E2D, direct running costs (for Private flying) are:
Insurance: $1400 per year (add some more for on line/flying school)
Engine: Rebuild after 2000 hours - between $20K-$25K + removal/fitting
Prop: Overhaul $1000, new $4K-$5K including fitting
Annual/100Hourly: anywhere from $500 (owner assisted) to $1K-$2K for a big one (never had one over $1500 so far - 80% have been less than $700).
Instrument check: $300 (small) to $600 (big) for IFR/NVFR
Get the STC for MOGAS and the wet hourly rate will be lower than other aircraft thus ensuring that your aircraft gets used more often - an aircraft used in a school should be getting at least 400 hours per year to make it beneficial.