PSA Auction Prices

PSA's Auction Prices Realised archive is a free, searchable database of historical sold prices for PSA-graded cards, aggregated from major auction houses, eBay and other public marketplaces.

For Pokemon TCG specifically, it is the best free reference for what PSA-graded cards have actually sold for over time, and is more reliable than asking-price-based estimates because every entry is a confirmed sale.