> Any given item in Amazon's database (where 'item' == 'something that has its own page') can have multiple variants on color, size, and other varieties. Each variant may have its own price. So if you want to sort items by price, which price do you use?

Use the lowest price, but include filtering options to exclude items based on the criteria that makes them different (e.g. show only Small shirts that are Pink or Yellow), and use the lowest price of the items matching the filters.