Andre wrote:I have seen one ... in a catalogue, I think

Yeah, that's about it.

And of course as I said, I have the disk + reference card so it DOES exist.
Atari Frog wrote:A "10" is usually assigned to a game that pops out of nowhere, is virtually unheard of , appears on eBay something like once in 7-10 years... or never
Yes, I agree with this actually. What I originally suggested, to have 10 only for games we don't know if they exist, is clearly wrong I see now. There should be a special rating for games that so far have not been verified to exist other than appearances in catalogues, ads and such.. The CURIOUS-scale uses "I" for this, I being Imaginary. Something like that would be useful..
I agree with what FastRobPlus says, a scale of 1-10 is a bit too granular. Perhaps the best thing to do is to get rid of all common games (1-5) and focus on the ones that are more or less rare? This could be accomplished by using a common rating for all common games, like "C". If a game is a 1 or a 4 isn't really interesting, both are easy to find.
My suggestion then, is that we focus on games rated 6-10, and leave the others for now.
My idea for a grade:
10 = unique or one-off items, e.g. 1 complete copy known (and perhaps some incomplete ones)
9 = less than 5 complete copies known
8 = less than 10 complete copies known
7 = less than 20 complete copies known
6 = less than 50 complete copies known
The question is then, how do we know how many copies are known? Well, one idea could be to add a feature to the database, allowing people to advertise that they own a copy of a particular game.. Basically allowing collectors to map their collections to the database. This is risky though. Some people want to be anonymous, and if we allow anonymous entries, chances are the stats will be corrupt, and the whole thing will be ruined as a result.
Probably the best idea is what you've been doing already I guess, letting people with knowledge in their particular fields to give input, and discuss titles on the forum?
Ijor, you raise good points. I do not think one should get involved with pricing at all, simply because such guides are always wrong, and they give the wrong impressions regarding rarity. Rare items can cost $1 while common ones can cost $100. There's no real logic involved, it's the supply and demand thing..