Is it cheaper to drop cargo than to land it?
That scenario only makes sense if your airplane stays at cruising altitude: although taxi and takeoff does use up fuel it's really the ascent to cruise that takes the most. You aren't really going to be able to drop cargo accurately from cruising altitude, so you'll have to descend pretty low, then you'll need to climb up again, and that would suck up lots of fuel and make it much less efficient a method of delivery. 

Add to that the weight and cost of the parachute mechanisms as well as the massive protective packaging the cargo would need to survive the jolt (2-3 Gs when it hits the ground) and the whole thing becomes pretty uneconomical. 

The military only air drops cargo when there's no other alternative - now you know why.