I don't think halon is illegal, just banned (similar to other older ozone depleting compounds). You can still find halon extinguishers... but as you noted, they are pricey and a time will come in the very near future that re-filling them will be all but damn near impossible.
I'd strongly evaluate what you actually want the extinguisher for.
1) If it is for camping, then I'd plan ahead and just take a 2gal yard sprayer with you when you camp and fill it with water prior to making a fire. There's not substitute for a proper fire ring and monitoring of a fire, but if you just want something more than dirt... I'd opt for something that you may already own and has alternate uses.
2) If it is for vehicle emergencies and you need to consider burning electrical, plastics, rubber, fluids, etc... strongly consider a halatron extinguisher or some newer 'clean agent' fire extinguisher. While they are pricey... you'll save in costs after the fire. In reading blogs about what happens to your vehicle electronics & solder from chemical agents... there's a BIG reason IT, server rooms, electronic manufacturers, and auto racers don't use chemical - damage costs from the chemicals typically far outweigh any small fire damage. If you're got a decent fire under your hood or in vehicle, you're probably going to be screwed anyway with a small 'clean agent' extinguisher regardless. If you tried to use a small 'clean agent' extinguisher on any decent camp fire that got 'out of control'... it would be of minimal help as there won't be enough of it to address the fire.
Finally - whatever extinguisher you decide to buy - make sure you know how to properly aim and use the type of agent & extinguisher you purchase. The application method of agent to a fire make a huge difference in how quickly and effectively the fire is suppressed.
