You could use the bucks toward PDGA membership IF the TD is willing to do it. However, the TD doesn't get discounted PDGA Memberships for resale, so they get a better deal moving merchandise rather than memberships for prizes.
They might do it, but will probably charge more than the 50 bucks for the membership. I don't know how much they are making on the selling of the disc. I would say at the very least 25%. so He might charge 75$ instead of the regular 50$.