+1 and Add stock use what you paid last

One more unit of a card, and the cost Add stock starts on, now follow the price you last bought that copy for.

+1 on the Cards grid books the new unit at what you last paid for that copy. It used to take the oldest unit still on the shelf, so a card you bought again at a higher price showed a margin it didn't have.
Add stock on a card's page starts its cost on that same last price. You can still type what this batch cost.