The backpack are "backup" items you've picked up from the previous dungeon, while the items in the locker are saved and will not disappear in subsequent runs. The price of items from the locker, backpack, and shops has never been consistent, unfortunately.
The backpack items are things you can "reclaim" that were sold when your previous run ended. That's why they're more expensive: Item value cost to equip, plus item value to reclaim.
Items you've lockered cost you their reclaim value when you locker them already, so their equip cost is just the item value + locker fee.