Its easier if you break the cost down into two parts Fixed and Variable. Fixed costs will be there from day one and won't change if you fly or not.
The basics for these are:
Re-payments ie Finance
Insurance
Hangared
Registration
Plus anything else that will occur no matter what
Variable cost are those that occur when you actually go flying:
Fuel
Oil
Maintenance
etc
You can then work out how many hours you think you will fly and times that by the variable cost. Once you have that you can add that to the fixed yearly cost and then divide that by the number of hours again and it will give you an hourly cost for that amount of hours per year.
If you looking at cross hireing the aircraft out you can than add in another section of what you will charge minus the hourly cost you worked out and then times it by the number of hours that it will fly and then you will see if it will make any money! Easy
Adam.