If you're flying domestic USA, on several major airlines the bike flies for the price of a bag.

Shipbikes.com isn't working right now, they were the best price around for me.

bikeflights was over $100 each way for ground shipping from Michigan to California a few weeks ago.

Either way you have to pack your bike in a box, might as well take it on the plane.

Edited to add:

I talked to the Delta ticket agent before my flight last week, she said the absolute limit is 52 lbs even though they advertise 50 lbs as the limit