The units used are the ASM and the RSM, available seat mile and revenue seat mile respectively. Any cost can be expressed by this measure.
Total cost per ASM/RSM
Labor cost per ASM/RSM
Fuel cost per ASM/RSM
Whatever you want.
The ASM is the product of an airline regardless of whether the product is sold (converted into a RSM) or spoils (stays empty). It's no different from any other company which produces a product for sale, they just use slightly different terms.