$15 each?! Nice find! How long ago was this?
I don't believe that there are any additional Fan Collective DVDs planned. There was rumor that a Romulan Collective was under works but I don't think that it panned out. There are BEST OF compilation DVDs being released (the most recent was last Tuesday with releases for TOS and TNG).
If you look around though you can find some season box sets for the same price as the Fan Collective DVDs. I recently found Seasons 1 - 3 of TNG at Wal-Mart for $15.00 each.