This was the process to determine if a CAT III installation was justified in Australia. This was a few years ago - I don't think it has changed.
If a CAT III installation was being considered at an airport the Board of Airline Representatives (which usually includes all the airlines operating through it) approaches Airservices with a proposal.
Airservices then determines what the costs are for installation, lighting, flight testing, ongoing maintenance and any other other extras.
Airservices then inform the Board of Airline Representatives what the costs will be over the life cycle of the installation and how much the navaid charges will increase. The Board then refers the matter to the airlines to determine if they still want the installation.
When the Board has received the replies from the airlines it then makes the decision on whether the installation will go ahead and informs then Airservices of the decision.
So the short answer is if the airlines want a navaid they can have it - but at a price. In the end the airline numbercrunchers make the decision on economics.