There is a Music Machine for Atari 2600 auction on ebay. The rare game sold for $5,250 last time it sold in November 2009 and bidding on this auction is already at $1,050.
One word of caution for potential buyers of this game. It looks like the seller has multiple sealed copies of Music Machine for Atari 2600. Here is a picture from the auction.
The game sold for so much the first time because it was super rare to find a sealed copy, but if this seller found a box of these somewhere there might be another three coming up for auction at some point and potentially even more.