Take the NBAA numbers and crunch in 60% if you know what your doing....multiply the costs by 3-4 if your the owner's idiot nephew, pal, golf buddy......who never managed a flight department before. Seriously.
An aside note...take what you consider to be normal downtime/unavailable aircraft and cut by 90% if you know what your doing...multiply by about ten if the flight dept manager doesn't like flying...puts the plane down for two weeks to change a tire, ect....