I might be cheaper to build, however parts to build with are alway a problem to get and sometimes compatability is an issue. You may be better served to have one built for you or purchase something already built. It my cost you a little more up front, but a little less problem and cost in the long run.
I'm new to this 'site and have been browsing around to get familiar with everything this site has to offer, I'm a DIYer, myself and naturally was drawn to this BIY page. being a big fan of AR-15's I figure this may give you idea's cygnus x-10:
It's not an AR-10, exactly, but with a few mods; it easily can be. Although, I think the plans have been taken off the internet, fortunately I copied all the plans. If you like I can have them scanned, and sent to you? Mind you, the lower in the link above was fully made on a sherline (tabletop) milling machine. So it may be relatively inexpensive.