Billy Williams was included in the Instant Impact insert set. The set includes the usual parallels and I've picked up three of the five. The two I skipped are the red (/10) and platinum (1/1). None of either has shown up on Ebay yet and I doubt they will.
The Blue is unnumbered
Black comes in at /299.
There are just 50 copies of the gold and I got mine for 99¢. I guess Billy doesn't have a very large collector base.