Hard to answer because there are a lot of variables starting with the type of plane you have (size, age, buy or build, VH or RAAus), who maintains it (you, LAME), cost of spare parts, cost of hangar or wherever you keep it, insurance, rego (more applicable to RAAus), fuel costs (avgas or mogas), how much you fly, etc. My plane is paid for and probably costs me roughly $100/hr to run in round figures although I've never really worked it out and it will vary markedly depending on hours flown. That's cheap in aircraft terms.