It may be diffrent in Europe under the CAA rules but here in the States it's a pro rata share of the direct costs of the flight.
*This part differs from inspector to inspector*
But the FAA has ruled that its the direct costs, so if you own the plane all you can charge is for fuel and tie down, you can't add in what it might have cost for maitnece.
Dan330: Correct under FAA rules (those are the ones that I know), but the rules say pro rata share, so that means for each passenger (you can have more of course in a C172 or larger plane), and for the time that they are onboard.
Also one thing you want to check out for those running commerical ops, if the pilot isn't a designated employee of the FBO, and you are not doing something that they have signed off on, then you might not be covered under their insurance as you would be in comes cases.
Basically that means I can't start flying passengers for hire to Key West, with a plane that I rent from my local FBO. Also that limitation is not only in the insuracen paper work, but it is also in some retal agreements.
Also one last note on what is considered commerical:
Commerical is only flight where the pilot gets some form the compenstation (direct or indirect), inexcharge for his flying services. If I am a repair man flying to fix washing machines and I use my own plane to get there that is not commerical. But if use Joe, my assistant as my pilot, he is recieveing some form of compenstation (I'm paying him I hope) then it is commerical, even though if you were flying yourself it would other wise not be.
Now remember ladies and gentlemen these are US rules that I am explaining and the interpetations vary among FSDOs. Please consult your local FSDO and an avation lawyer, before you begin any commerical flying operation.
This information is provided free of charge with no warrenty what so ever, implied or otherwise.
Had to put the CYA stuff on the bottem.
At worst, you screw up and die.