Price for a Colt AR is $100.00.
It usually takes around six months to get the letter unless you pay extra for an expedited letter.
View the site to see some sample letters.
They'll tell you in what configuration the rifle was when it was shipped, the date shipped, how many rifles OF THE SAME TYPE were in the shipment, and who it was shipped to.
In most cases, with an AR the place shipped will probably be a distributor.
Most probably, the Letter for an AR isn't going to tell you much you don't already know, unless someone has changed things on the rifle or it was a law enforcement rifle, and only then if the rifle was direct ordered by a law enforcement agency.
Most police departments no longer direct order, they usually buy from a distributor or local FFL Dealer.