The rate quoted for Schoolhouse Nine is for nine holes, eliminating that one.
Replay is $10, so you're still getting 18 holes for $30.
The original post never truly spelled out a course must be 18 separate holes, although it is somewhat implied and most responses hence have been for 18-hole courses.
I'm inclined to allow 9-hole courses with replays that come to $30 or less, but remember we are looking for the
best course at that price range. It'd better be a pretty darn good 9-holer....