Are you talking about the Dave Christensen backglass that was made for the Williams Big Deal? That's just a backglass as far as I know, put on an existing machine. Rumor has it there are at least 20 of them in existence. I have one as well.

I know the EM version is super rare, but what about the solid state version? Is it just the rarity that is the factor? I have a SS version of Charlie's Angels that is in gorgeous condition but it's going to require a new set of boards so it's been sitting - I wasn't under the impression it was that valuable.

Has anyone ever seen an EM version of Genie? I heard that may be a similar situation with only a few produced...

Regarding rarities... I think there are other games that are much more rare than people think.

For example I recently came across a game and when I went to IPSND.NET, there was not a single instance of it being owned/registered in the database. While that's not necessarily inconclusive evidence that it's super rare, there's enough data out there to indicate that when you see a significantly lower number of game serials known, the game may be much more scarce.

As others have commented, what makes a game "rare" can be a combination of many factors. Certainly in some cases (*cough* Varkon *cough* it can be because the game is bad, but it can also be because of political and legal issues, or in the case of Pinball Circus, it was too expensive to make and shelved. Many of the Atari pinball machines were made in low quantities because they proved to not be very reliable on location due to certain design deficiencies. The games may be fun to play, but were high maintenance. Lots of reasons why a game may be rare. In the end, value is going to be somewhat arbitrary. There are some people who collect for rarity and other who do it for the enjoyment of the game. But rarity is definitely not a consistent indicator of value.