Here is my experience with 640+ hours so far on my RV-9A with a Lycoming IO-320.
My fuel cost per hour over the last 5 years (tracked in a spreadsheet, since I'm a computer geek) has been $26.75. I use around 6 gallons per hour of 100LL in my flying. Never tried autogas, since everything in California has ethanol.
I do an oil and filter change 4 times a year. This costs me about $300 per year, includes getting the oil analyzed.
The only other "maintenance" costs I've incurred have been tires and brakes.
At various times for annuals I have replaced little things like the crank seal on the engine (finally got it to stop leaking oil), touched up paint, and done some electronic upgrades (fix flakey EGT probes, replace the ELT battery, new 2020 GPS, knob panel for the Dynon Skyview). The P-mags use automotive spark plugs and that costs me $24 per annual to replace all 8 of them.
This year my annual is coming up on 5 years, so I will be taking a look at possibly replacing my alternator belt. I think the hoses are good for longer than 5 years.
So I'm 1/3 of the way to TBO, and hopefully this trend will continue. When it comes time to overhaul, I may just swap an IO-360 in her.