according to the bank of england – only a few pence each. The print set up costs if/when they change note designs are pretty significant, but the sheer size of the print run cancels that out.
Back when there was that big raid in Northern Ireland where millions were stolen, one of the factors that lead to it being returned was with such a significant sum stolen (the cash stolen was the NI design of note) the notes can simply be withdrawn and replaced with a new design, – easy for the general public to deal with as people rarely have more than a wallet full of notes to exchange for new, but impractical with large sums, leaving the robbers with millions in what would suddenly become toy money – meanwhile the sum that had been lost could can then magically re-appear in the bank as new notes. So the thieves would have stolen nothing and the bank would have lost nothing.