Buying half a tractor is a figure of speech.
Being realistic an irrigation system for a 9 hole golf course still requires a pump house, irrigation controller, it is certainly not half the price of an 18 hole system. You can make the case equipment lasts longer, but you still need the same sized machinery store if you run all the kit. If you want to be a good club, you still have a pro, secretary, bar, staffing costs won't be half and as soon as you slash the staffing level the quality of the operation drops and so does the amount you can charge so eventually finances bite too much and typically you end up with a 9 hole course at £400 per year subscription ran by a couple of people.
This does not matter if you are downsizing from an initial 18 as you would already have it, it will delay the inevitable though.
The main negative for a 9 hole course is the perception by most golfers wanting to play 18 and 18 holes is a round of golf, going round twice is a big no no for many. Most people will join the 18 hole course.
Housing generates lots and is a different thing altogether.
A lot of clubs could have pockets that release a few acres of land and in some areas a few acres relates to a few million. The land needs to be next to roads and close to services, but losing a couple of holes and finding some alternatives might be a good route for some clubs. That way they stay 18.