Opensea contains a ranking page for the top NFT collections. The rankings are based on volume, floor price and other statistics. When viewing the rankings, you’ll be able to see the current lowest price that an NFT in an NFT collection is going for. If you click on an NFT collection’s page on Opensea, you’ll be able to see a chart of that collection’s average sale price, trading volume, and number of sales over a selected time period.