I remember some old sci-fi book or short story (don't remember which one) that had a spaceship with a separate spinning section specifically for a toilet.

You would enter it, activate it, wait until it accelerated to a certain RPM, do the thing, then deactivate it and it would decelerate until it is stationary relative to the rest of the ship again.

I wonder, how expensive it would be to build this for real.

The rotation mechanism could use a flywheel. Let's say, an electric motor spins the toilet section and the flywheel in opposite directions. So that the rest of the ship is not disturbed.

Size and weight are obviously issues, I just wonder how much would be the overhead. I wonder if the real spaceship designers considered this possibility.