You pay for a finished wall, not for a bag. The cost of one square foot is the price of the bag divided by its coverage.
From each kilo, SIG-MR covers 1.08 sq ft at 13 mm. A plaster with 14 to 15 sq ft from a 20 kg bag covers 0.70 to 0.75 sq ft from each kilo. Lower coverage means more bags to buy, to transport, to carry up and to mix.