Back when I worked at Tire Kingdom, our P/S flush worked like this:
A machine would vacuum fluid out the reservoir the same way a turkey baster could. Fluid for flushing would get pumped in and pumped out. We were always given 2 quarts of the stuff. Next, we pumped in synthetic power steering fluid, pumped it out, and pumped it back in, 2 quarts of synthetic fluid.
So if it gets expensive, it makes sense.
Still, I would much rather just suction out old power steering fluid and put in new power steering fluid if it was my car.