First of all, I think it is an execellent idea that you are thinking of the safety and assurance of your computer hardware and networking equipment. Some companies, and the employees responsible for these duties of disaster recovery and protection have to make do with what they have sometimes. Your best bet is to look into getting some sort of insurance policy that your company can afford, for all the valued equipment that you want insured and protected.
I'm not entirely sure on the Halon systems, as they have gotten out dated in the past 5 years, and yes they are very expensive, and once discharged, are costly again to setup and certify.
I believe there are other alternatives to halon now, but may not be any less expensive, suchas other discharge systems that are ceiling installed and discharge a oxygen combating agent of some sort to put out an electrical fire with the least damage possible. Once again, certified insurance companies could recommned more about such a system. Many buildings, containing server rooms, that used to be used for something else (such as a storage closet) have standard sprinkler systems distributed through out the building for general fire safety code reasons. Possibly you could have building maintenance disable a range of sprinklers, not sure.
One of your best bets at this time, is to invest in Computer room specific fire extinguishers, which contain dry agents/chemicals, and are not liquid foam type. These extinguishers could beplaced in, or very near your server room.
Other than that, you could survey any electrical risks within your server room yourself, or with a company electrician, to weed out possible fire risks, like jam-packed outlets or surge supressors.
Best of Luck!
Paul M. Chavez