I was reading in the papers how some-airline-or-the-other increased their prices for extra luggage, citing increased fuel costs.

Now I'm a bit skeptical. Using the (wrong) Bernoulli-effect explanation of lift, I get this:

More luggage$\implies$more lift needed $\implies$ more speed needed$\:\:\:\not \\!\\!\\!\\! \implies$more fuel needed. At this point, I'm only analysing the cruise situation. When the plane is accelerating, this will come into effect, but more on that later.

Now, I know that the correct description of lift involves the Coanda effect and conservation of momentum, but I don't know it well enough to analyse this. Also, there will be drag forces which I haven't (and don't know how to) factored in. I can see that viscosity must be making a change (otherwise planes wouldn't need engines once they're up there), but I don't know how significant a 1kg increase of weight would be.

So, my question is: Are airlines justified in equating extra baggage to fuel?

Bonus questions:

• If more baggage means more fuel, approximately what should the price be for each extra kilo of baggage?
• What happens when we consider takeoff and landing? Does a heavier plane have to use a significantly large amount of fuel?