All card pricing on the pkmn.gg platform comes from TCGplayer market prices. When you see a price under a card, this is the near mint market price of the raw, ungraded card.
Click on a card to view the Card Details of any card, then click the Price tab to see a history graph of available pricing data.
Across the top of the graph, you can switch between the Last 30 Days, Last 3 Months, Last 6 Months, or Last Year of pricing information.
Below the price graph, you can also see the price change for a given time period and the percentage change.
If you want to see the pricing of a special variant such as Pokémon Center Stamp, click on a card to view the Card Details. You will see any data available for other variants and their pricing. You can view their price change history in the Price graph view as well.
As you continue to track your collection on pkmn.gg, you'll see your Total Estimated Collection Value update.