Complex sentence: It might be questionable to assert that the cost of 10 extra people on the sales force are an incremental cost or an overhead cost, since the wages for these people are both overhead and incremental.But staff needed to keep the shop operational are mostly considered as overheads.
Split into simpler sentences: It might be questionable to assert that the cost of 10 extra people on the sales force are an incremental cost or an overhead cost, since the wages for these people are both overhead and incremental. But the staff needed to keep the shop operational are mostly considered as overheads.