Rare in my mind would be similar to what Mr. Black says, which would be 75 chips or less. But you can have 'rare' as in hard to find chips, despite the number that were produced.

For example, the three-chip collection from the Paris (the pin-up gals) would definitely be 'rare', as you hardly see them advertised on eBay. Rarer would be having all three chips with the same identical LE number (mine are all #60). But they're still part of a 500 chip run, so value would be highly variable. (Admittedly the value of the set that I have would probably be higher than three random ones... but what do I know?)

In the longer run, value is what you view it as. If you appreciate your collection, then rarity is subjective. It's nice having a chip that might be one of maybe a dozen or two, but it's equally nice to have a chip that is beautiful and a work of art even if that chip might have been minted in the thousands (or just a thousand, like the Main Street Station Mucha collection).

But as always, your mileage may vary, warranty void in Vermont.