Would a larger Lion order cut unit costs?

How much would the cost fall if Britain and the Dominions ordered about seven similar Lion-type ships?

Only modestly, because several yards would build the ships at much the same time.

An order for seven similar ships would be spread across several yards and built largely in parallel. The builders would complete them over a short period instead of sending all seven sequentially through one yard. That limited the repeated production experience associated with a serial building programme. Buying more ships could still reduce costs somewhat, but the expected saving is small.