As a golfer, I find Spain too hot to play in the peak Summer months, even using a buggy, therefore there has to be more on site than golf. Near to the beach is a must. That is why La Manga is seen as the place most resorts aspire to, lots to do apart from golf, with its own beach (albeit small). So to me a place in the boonies far from anywhere apart from an airport (not yet built, but noisy?) does not appeal in the Summer.
In the Winter, a 9 hole course does not appeal either. If there was nothing else apart from a 9 hole course, I just would not go there. However, Murcia has loads of courses available or under development, so lots of better choice.
Also consider this, the number of visitors to Spain has declined for the third year running and the number of planning applications for houses has doubled to 1.4 million in 2006 (source Greenpeace report). So in terms of rental you are facing increasing competition for a declining market with a product of less appeal than the competition.
Would perhaps you ask your agent to justify any claims for rental potential.