Brock's Onix [CoroCoro].

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. Brock's Onix [CoroCoro] 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. $6,000 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.