Digital Press has a good scale for rarity.
This is copied from Digital Press' online guide.
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There is a scarcity value commonly appears next to the dollar value, and it represents a scale of 1 through 10 with 1 being the most common and 10 being the most rare. The following is the legend to use when viewing these rarity ratings.
* R0: Titles listed with a rarity of zero are simply games that were never released or require more research on our side. You’ll see this listed on ALL “Rumor Mill” entries.
* R1: These are the items you can find or BUY anywhere. When you find a bunch of games, these WILL be in the pile.
* R2: You can find these titles just about anywhere, too, but they’re slightly less common, perhaps in certain areas.
* R3: You’ll find these items through the proper channels (other collectors!), not as common “on the street”, but fairly common on the internet.
* R4: Things get a little harder here, as a title rated as such could have a lower production run, or other factors that pull it out of the common bin, but nowhere near rare. You can start calling titles from herein UNCOMMON.
* R5: The hunt begins. Five are the kinds of item you have to look for, but definitely always in reach.
* R6: If you see a six or greater, at least one of us had a problem finding the title. You call this VERY UNCOMMON if you so desire, or BORDERLINE RARE.
* R7: RARE. A true collectible! Sevens and up represent the “hardest” 25% of the games listed in this Guide.
* R8: We consider these the items you’ll never find by chance (except in those legendary lucky finds!). There are a few eights in this Guide that we haven’t gotten our grubby little hands on... yet.
* R9: The centerpiece of a collection make up the nines. If you ever hope to BUY one of these, prepare to write out a big check.
* R10: Not only is this the centerpiece of a collection, but a ten also means you could be holding a one-of-a-kind (certainly one of no more than a dozen or so) there - kind of like owning a 1952 Topps Mickey Mantle, or Issue #1 of DC’s Action Comics.
We include the scarcity ratings to address the need for a rating system across platforms. What we found out through many letters, e-mails and other excellent feedback was that prices provide a good baseline but were not addressing how difficult a cartridge might be to “find”. So we’ve done both for you. The scarcity rating provides a very simple one to ten system (a longtime DP tradition). “1” is the common doorstop. “10” is the item your friends want to steal.
Note that a "2" for the Atari 2600 should be just as easy or hard to find as a "2" on the Neo-Geo system. It's all relative across systems, not within them.
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I checked Valkyrie Profile and that came up as a 3 in their database. Intelligent Qube is a 5 and Panzer Dragoon Saga is a 6. They really don't have a lot of data on the current gen, but a lot of the games j-fever posted would likely come up as R2-R4. They're still somewhat common on the internet. Take a look at some of the items rated R8 through R10. Some of that stuff is very limited, or for the R10s, prototypes.