Mew.

Why this card has no market price

The market prices on this site come off TCGplayer, where hundreds of sellers list the same product and the daily average is a real market. Mew is not a TCGplayer product and never was, so no such average exists for it. The ladder above is built from recorded sales instead: what copies actually fetched at each grade, whenever they last changed hands.

Read it that way and it holds up. A market price moves every day, a sale price moves when a copy sells, and at this level that can be years apart. $6,600 is what the last gem copy is on record for rather than a quote you can act on this morning, and the next sale will land either side of it. Only 6,837 copies have been through PSA in any grade, which is the other half of the story: the price holds because almost nothing comes up.

The sale figures on this page come from the PriceCharting listing for Mew #151, which tracks them grade by grade.