I have found it pretty easy to calculate operating costs. If you know what your insurance is gonna cost you, and the fuel burn of the airplane, the only unknown is repairing stuff when it breaks or comes up as a squawk during an annual. Everything else can be pretty easily estimated. The main thing is how many hours are you gonna fly-- all your per-hour fixed costs (hangar, insurance, annual) go down the more you fly.
Example for a simple 150-horse airplane:
hangar $3K/year
insurance $1K/year
annual & maintenance $1K/year
fuel $6 per gallon x 8 gph= $48/hour
oil/filter $60 every 30 hours = $2/hour
engine reserve $20K/2,000 hours = $10/hour
So per hour costs are about $60/hour. Now let's say you fly 100 hours a year,with $5K of fixed costs that works out to about another $50/hour, making a grand total of $110/hour-- $100/hour if you don't count the deferred cost (engine reserve).

Holy crap, I'm selling that bank-breaker & buying me a Harley!!!