Monocock, our figures aren't so different in some ways (hanger and insurance are almost identical). The reason we come out with a different bottom line is:
1) Fuel: I'm guessing you've costed about 6 gallons per hour, my figures were based on 10. (5k difference)
2) I've not figured in any appreciation (8k difference, see below).
3) I've figured in the cost of funds (4.8k difference).
I can certainly see where you're coming from, but assuming we can both add up properly (or Excel can, which is what I used) then there is no mystery to it - you get different answers if you make different assumptions and when you're only dividing the final number by 100 it doesn't take much to have a 10 pound/hour difference in the bottom line (about a thousand pounds I'd say, which isn't much based on points 1 to 4 above). Above I've accounted for 18k worth of expense over three years - 60 quid an hour.
I am very surprised to hear that an aircraft on which you spend no money other than basic ongoing maintenance (i.e. no avionics upgrades, repainting, etc) appreciates 20% over three years having had three hundred hours added to it. Is this normal? Could it not be explained by changes in economic climate or buying at a bargain price and selling at a good one?