It depends on the airline. Some charge extra no matter what, and others will not charge if you include it with your luggage allowance and meet no higher than the maximum size of a piece of luggage.

What I did was use the Performance luggage, and I took the bike down as much as possible, and it worked for me. I've also had a luggage made for me using cardboard, and that also helped keep me within the size restrictions. After doing that for 3 times, I got a Bike Friday, which is a folding bike, so I'm always within the size restrictions, and I don't get charged since it has it's own luggage case.

Koffee