I want a system that has the following requirements:

-Be able to install easily on a any kind of multicopter (sub-750g)

-Be lightweight (Under 50g would be great, I still need to figure out how light this actually needs to be)

-Be cheap (If not under $10, at least under 30 bucks)

-Be totally independent from the main system: I want something that deploys the chute when it detects an abnormal drop in altitude (or a g-variation, I have both sensors)

The first thing I have to figure out, is how big the chute needs to be. If I follow a matching table (on page 17 of this PDF: http://www.planete-sciences.org/espace/publications/techniques/parachutes.pdf ), I need a spherical parachute of 43.8cm in diameter to slow down a 1kg body at 10m/s. Now, we will be braking only 750g, but I want it to be slowed down a bit more, so I'll round  this diameter up to 45cm.

With this in mind, I'll first make a mock-up of the chute using simple trash bags, to get a feeling of how big the compartment needs to be.