Every Mareep card we price, most valuable first. Prices are what copies actually sold for, updated daily.
What a Mareep card is worth
The most valuable Mareep on this list is Mareep (Toys R Us Promo) from Miscellaneous Cards & Products at $100£75.26€88.07, and the 29 priced copies across 27 sets are worth $474£357€418 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.
Mareep is printed in Japanese as well: 35 cards across 30 sets, the most valuable of them Mareep from McDonald's Pokémon-e Minimum Pack at $60.87£45.81€53.61. 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 Mareep sold for $1,724£1,297€1,518. 1st Edition and 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 Mareep 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: Miscellaneous Cards & Products, Aquapolis, EX Team Rocket Returns.