The following was listed on gunbroker.com, starting at $639 with no reserve.  You're the experts, what do you think?

GET ONE WHILE THEY LAST!!! These sporters are NEW condition Built with Colt military parts and a DPMS military Spec receiver. You heard right, Colt parts and DPMS receiver!! Feature's 20" heavy barrel with muzzle break, A-2 carry handle and all black synthetic stocks. Very nice military spec rifles at a great price. Get the name brand quality without the hefty price. They each come with a COLT 30 round magazine.

WTS a Rolls Royce Engine in a Ferrari body with Porche mags....

for a hundred or so you can bet a COMPLETE Bushmaster AND their customer service...

Go for the Bushy!!

All in all,  not a bad price for a "Frankengun" Will probably function just fine, esp. if all parts are new. Where you're screwed with a Frankengun is that if it don't work, you have no recourse but the kindheartedness of the dude that you bought it from. Me personally, I 'd drop the extra Ben Franklin and change and go with a factory Bushy for just that reason, ESPECIALLY if this your first AR.

I suggest you check shotgunnews. Sometimes you will see full page ads of dealers selling bushmaster AR15s. You might be able to pick one up for $600-700.

Prices that low are almost always from wholesalers.  You'd still have to find an FFL who will order that for you and pay whatever they charge for the service.  The average guy on the street isn't going to get that kind of pricing very often.

Thanks for the input.  All in all, it doesn't sound like the way to go.  Guess I'll look for something in a virgin setup.