What actually moves a prospect card's price

Prospect card prices do not drift upward with good play. They sit still for months and then move in a day, because the thing being priced is not last week's box scores — it is the market's belief about where the player ends up, and that belief only changes when something changes it.

This site watches for those moments rather than for daily movement, and it is worth knowing which ones they are, because the list is shorter than most people expect.

Step changes, not drift

A promotion to Double-A re-rates a card far more than a fortnight of good box scores. So the detection compares two snapshots of a player and looks for a step between them — and every event names the two values it moved between, so the claim can be checked rather than taken.

A missing value on either side produces no event at all. “Level unknown → Double-A” is a data import, not a promotion, and calling it one would put a fake catalyst on a card at the exact moment somebody is deciding whether to buy it.

The six things that count

Notice what is not on that list: a hot week, a prospect-list ranking, a highlight going around, a new parallel appearing on the market. Those move attention. The list above is what moves the belief.

The harder question: does the price already know?

Every event above is public. By the time you have read that a player was promoted, so has everyone else, and the number you are looking at may already contain the news you are buying.

So the more useful comparison is between the baseball and the cards over the same window — how much the player's trajectory has improved against how much his cards have actually moved. Three outcomes, and the middle one is the common one:

• Early. The baseball is improving faster than the cards have repriced. This is the only one of the three that is an opportunity rather than an observation.
• In line. The cards have moved about as much as the baseball justifies. Nothing to do; the market is working.
• The cards ran ahead. Prices moved further than the play supports — often after exactly the kind of event in the table above. Buying here is buying the news twice.

And separately from the direction, a blunt warning: when the trajectory is genuinely good and the cards are already up 40% or more over the window, the price plainly knows. That is not a reason never to buy — a real re-rating can keep going — but it is a reason to stop calling it undiscovered.