No energy, as such, is required for stable, level flight. Since the aircraft is not changing it's height or speed, no work (in the physical sense) is being performed. So that kind of makes the whole discussion nonsensical, doesn't it?
The wings produce lift, used to counter gravity. This is no different from you sitting on your chair right now - the chair is acting on you with a force equally big but in the opposite direction of gravity. That force prevents you from falling. However, if your chair is not lifting you up or letting you down, there is no work being performed.
If you now also decide to scoot your (rolling-)chair along in the corridor, according to Newton, it would want to continue rolling along the corridor with unaltered direction and speed. However, friction works to slow it down, so you use force (your feet) to maintain said speed. Again, no net work is being performed as long as you maintain speed. You do work, and so does friction, but the physical resultant work is zero. There is no net change in energy.
If you could create a chair with frictionless, or low-friction wheels, that would make YOU perform less work to maintain speed, because the chair is doing less work the other way. So it would be beneficial to you that the chair drag was lower for the same chair-up-force (the one that keeps you sitting), and you could calculate a ratio for that, and call it....
Edit: and Barit1: The net lift on a 200-ton airplane is not 200 tons in climb or descent. Lift is defined as the components of forces acting normally to the flight path, and so if the flight path is not normal to the force of gravity, lift and gravity is not 180 degrees apart, and for them to equal out, they must be different, and have a lift/drag component as well. This is easily provable by asking how much lift is required for a 90 degree (vertical) climb? None! But a whole lot of thrust. Again, in a vertical descent, all the lift in the world would not make you decrease your rate of descent, only drag would.
Edit: perhaps I didn't make myself clear (perhaps I don't have it figured all out, that's probably it, actually). I was starting the post out as a discussion on work being performed along the normal axis, ie vertically, and the thrust/drag component aside. And then I obviously went away from my original thoughts. I haven't edited the contents above thogh, because of the posts below.