Well, since nobody seems very helpful tonite (it IS near full moon, isn't it??), I'll give it a crack. Many here would tell you to take that offer of doubling your $4 fast. IIRC, they are selling for around $800 new?? Sounds like accuracy is usable on most who've ventured forth with range reports. Most parts on them do not interchange with standard AR-15 parts, as they are not really an AR-15 - but they sorta look like one. Very light, and since Bushy bought out Professional Ordinance (the original manufacturer of the Carbon-15's), they seem not to have given many folks problems by and large. Nice and light for carrying all day.
That said, you can build a poly lower AR that weighs in at about a pound over the Carbon-15's - even more with a standard forged lower.
Do a search for Carbon 15 in the search option above - it is asked here every month or so, and has already been responded to frequently.