The incremental cost refers to one shuttle flight divided by pounds.
I think what OP was asking is - given shuttle already had a mission with some spare payload capacity, how much did it cost to bring one more pound into space? I think this question has a relevancy to the fact the printer was surprisingly heavy and came with a lot of paper. I'm not sure if there's a good way to calculate this though.
Its a bit of a nonsense calculation anyway. Fundamentally the shuttle's engines had a maximum thrust they could generate, and there's no amount of money that could easily change that. That dictates the maximum shuttle + crew + payload weight and it's just a question of how you parcel that out. Include a heavy printer, and you constrain the maximum crew and/or payload. (There was one flight, recently covered on HN - https://en.wikipedia.org/wiki/STS-93 - where they flew with fewer crewmembers because the payload was so heavy).
I think what OP was asking is - given shuttle already had a mission with some spare payload capacity, how much did it cost to bring one more pound into space? I think this question has a relevancy to the fact the printer was surprisingly heavy and came with a lot of paper. I'm not sure if there's a good way to calculate this though.