Rej
Surely the normal landing, parking fees etc would suffice- afer all an open airfield is an open airfield
OK how about this... Imagine you're sat at your military airfield (not worrying about the cost or commercial aspects) The squadron cancels night-flying but you have accepted two additional civil moves on the basis that "you were going to be open anyway"... then while you're waiting for these to pitch up, another aircraft comes on frequency, without having made prior arrangement, and says "ah, you're open...." Would you be so happy to be "providing a service" then?
Sorry you can call it business, or deterent if you like. It may seem logical to accept traffic if you're open, but I feel a line has to be drawn somewhere. If asked to provide a service beyond your normal operating hours you are entitled to charge what you like... and if that charge is unacceptable, they always have the choice to go somewhere else...