The main get from the Instaflator is not the bag itself, but the valve. It comes apart and can be attached to any bag, so you can put it in the corner of a pack liner or just about any kind of bag you'd like. I haven't measured it in a while, but I think my Instaflator valve, trimmed down to just the Therm-a-Rest fitting, weighs about half an ounce. I also cut my bag in half bringing the total kit to just a hair over an ounce. The leftovers can later be substituted in if my current bag pops.
For what it's worth, my Instaflator has about 30-40 fills on it with no problems.