What is the value of a 1950 B US 20 dollar bill?

Despite its age these bills are not especially valuable. As of 09/2010 a circulated one might retail for $23-$25, while a nice uncirculated one might bring $35 or so.

In 1950, several different series of 20 dollar bills were in circulation. A series B 20 dollar bill is worth around 30 to 50 dollars.