You may be right. Even if they started out as a 5% or less error population, the errors were culled out right away. It might have been like the Caesars errors (3-inserts vs. 4-inserts) in which Caesars destroyed the errors they found.

I was fortunate enough to get a single-error and a double-error Mirage "Poker" chip, but it was right after they came out. I do think the double-error should be valued quite a bit higher than the single-error, from a rarity standpoint, but there's a ceiling on how much of a premium chippers will spend on any error, unless it's unique or exceedingly rare, IMO.

Regarding your Mirage poker room experience, I had a similar experience at the Venetian when the $5 rack chips came out. They were supposed to be H&C mold, but some were made using the Venetian HOUSE mold (reserved for $25's & up, it appears). These are valued now at $5 face value for the H&C, but $100 for the HOUSE mold.

The 2 Venetian $5 rack chips (pics from MOGH / Chip Guide):

I bought rack of $5's at the cage, went though it without finding a single error, cashed it in, and a bit later bought a second rack. The cashier did sell it to me, but remarked something like "All the $5's have been picked through for the good ones." Sure enough, no errors in that one either. Word travels pretty fast!