What is the cost of pumping all the air out of a tunnel hundreds or thousands of kms long and the machinery to do it?
Nobody's talking about pumping
all the air out of the tube, just some percentage of it. Enough removed to reduce drag, enough left over for the engines to work.
ie. Some amount of air is
needed inside the tube. The exact amount needed is still being tested and figured out via all these prototypes that people are building.
Your question is impossible to answer right now because the difference in cost/difficulty between removing 90%, 95% ... 99% of the air is orders of magnitude.
It may be that maximum efficiency is with only 80% removed, nobody's sure yet. 80% is a
lot easier/cheaper than even 99% vacuum.