I've been looking around a little, researching fire bottle systems. There are a lot of options for extinguishing agents.
That $400 option is a foam system, which can do a lot of damage to electrical and mechanical components, and isn't easy to clean up. Not to mention you don't want foam near your lungs, so a no-no in the cabin.
Dry chemical is OK, most just vacuum up, but again if you don't manually remove it all it can corrode and degrade components. Not good for lungs either.
Halon(and Halotron) leave no residue at all, but may be carcinogenic.
The last one, and one of the most expensive, is Novec 1230(and FM200, almost identical). Clean agent, no residue, and no documented ill effects to eyes, skin, and lungs.
That same $400 system but with Novec 1230 is $700. I think that would be my choice.
These systems are available in metric sizes: 2.25 kg is 4.95 pounds, 3 kg is 6.6 pounds($800 for the above system with larger 3 kg tank). Some sanctioning bodies have a 5 lb. minimum hence the very slightly larger 3 kg size, as they won't let you slide for 0.05 pounds.
2.25 kg:
http://www.lifelinefire.com/zero360fia225kgnovec1230firemarshalsystem.aspx
3.0 kg: