Every Diancie card we price, most valuable first. Prices are what copies actually sold for, updated daily.
The same Pokemon is also printed as Mega Diancie, priced separately because the game treats them as different cards.
What a Diancie card is worth
The most valuable Diancie on this list is Diancie Prism Star (Naohito Inoue) from World Championship Decks at $23.32£17.55€20.54, and the 15 priced copies across 14 sets are worth $94£71€83 together. Most of that sits in a handful of printings: the older the set and the scarcer the print run, the wider the gap to the rest.
Diancie is printed in Japanese as well: 15 cards across 11 sets, the most valuable of them Diancie from PokeKyun Collection at $21.96£16.53€19.34. Switch the list above to Japanese to read those, or to Both to rank the two catalogues against each other.
The highest PSA 10 on record for Diancie sold for $731.15£550.30€643.90. reverse holo printings are priced separately from the standard card, because they do not trade at the same money.
If you are trying to identify which Diancie you have, the set name is printed on the card and the collector number sits in the bottom corner. Open the card here and the page carries every printing of it, the graded prices, and whether grading the copy is worth the fee. Sets to start with: World Championship Decks, XY Promos, XY - Phantom Forces.