Some courses I frequent do indeed have a 9 hole rate. Mostly for walkers. Cart fees are what they are, regardless of the number of holes played.
My home course charges a flat fee for 9, or 18. Walkers pay $12. Riders pay $25. (non locals may pay more) Those fees are even lower in the hot, summer months, during their "early bird" and/or "twilight hours" promos.
Another course, just a block away, charges $10 more across the board than those I listed above. 9, or 18, it does not matter.
I know some guys who take inappropriate advantage of theses rates and play all day long. Maybe 54 holes, or more.