| Depends on the bass, whether it's in a heavy case, whether you are near a primary hub for one of the shipping companies, whether you're insuring the package, and how far you're shipping it. It can realistically cost anywhere from $25 to $100 to ship a bass within the US.
There are ways to minimize the costs, but they almost always involve compromising the safety of the bass en route. The only exception is if you can unbolt the neck from the body, and if the buyer is OK with you doing that. Unbolting the neck means you can fit it all into a shorter box, which will cost less to ship. | 
