WOTC-era Black Star promos (1999 to 2003)
The first 53 Black Star promos covered the Wizards era, numbered 1 to 53. The most valuable include #1 Pikachu, #4 Mewtwo, and the holo movie promos. Most are cheap; a few cards (the holo Mew with the gold W stamp, for instance) trade in the hundreds in PSA 10.
Black Stars from this era have a black star printed in the lower-right of the card where a set symbol would normally go. The number on the back identifies the promo.
Nintendo Black Star promos (2003 to 2010)
When Nintendo took the Pokémon TCG license, they continued the Black Star promo numbering. These cards run from EX1 through DP56 and HGSS01 through HGSS22 across the era. Mostly mass-distributed event cards with low collector demand, but the holo Lugia and Ho-Oh promos hold modest value.
Stamped promos (Staff, Pre-release, Build and Battle)
A regular set card with a foil stamp applied to it. The most collected stamps are STAFF (event staff only), PRE-RELEASE (Build and Battle promos), and BUILD and BATTLE (modern Build and Battle deck inserts). A Pre-release stamp on a common card adds little; on a rare or holo, it can double the price.
Worlds promos (2004 onward)
Top-finishing players at Pokémon World Championships receive numbered, signed deck reprints featuring their winning decklist. These are extremely scarce (often 100 to 250 copies total) and trade in the hundreds to thousands depending on the year and the deck iconic status.
Trophy cards (rare, expensive, often misattributed)
A small number of trophy cards exist from major tournaments: the Tropical Mega Battle, the Super Secret Battle, the No. 1, No. 2, and No. 3 Trainer cards from the Japanese Tropical Mega Battle 1999. These trade in the five and six figures because populations are 7 to 50 cards total. The Pikachu Illustrator card from the 1998 CoroCoro art contest is the most famous example, with 39 copies total and a record sale north of $5 million.
How to value a promo
The framework: distribution channel (how many were given out), age (older is generally scarcer), holo or non-holo, condition, and whether the card has subsequently gained iconic status. A modern McDonald promo from 2022 distributed in 50 million Happy Meals will never be valuable. A 2001 Best Buy Mewtwo distributed at 1,000 copies will trade in the hundreds. The math is always population versus demand.