What is the value of a confederate 500 dollar bill?

The vast majority of "Confederate" bills are actually replicas made for sale in gift shops at Civil War sites, or for inclusion in souvenir sets.

Starting during the 1970s all replica bills were required by law to have the word COPY on them, but millions were printed before that time. Most were printed on artificially aged paper using photoreproductions of genuine bills so they can be hard to tell from the real thing unless you look closely at the signatures - the ink is flat and on the surface of a replica, while it is a slightly different color and more deeply impressed on a genuine note.

The Related Link has a list of serial and plate numbers found on the more common copies; you can compare those to the numbers on your bill. If they don't match you should seek out a dealer or appraiser who specializes in Confederate currency and have the bill examined in person.