AR-180 "A"s (if you're referring to the Sterling, Costa Mesa or Howa models) are prohibited because they are listed as such under Former Prohibited Weapons Order #13 thus making them 12.5 prohibited firearms. Them being prohibited has nothing to do with being "FAL-based" (which they're not), able to take full auto parts or any other such nonsense people like to spread. They're prohibited because they were specifically named as part of a list when the various prohibited firearms OIC's were passed. Probably because in 1984 a large, unknown man stormed an LAPD police station and, along with the also named prohib SPAS-12 shotgun, killed over a dozen police officers searching for Sara Connor.
The "B" models aren't especially rare in Canada, but they have been out of production since 2007 or so. This is making them harder to find as more and more gun owners are not parting with them. What they're worth is what a buyer is willing to pay for it. I wouldn't pay more than $1100 and I own several of them. I've seen them go for $1500 at gun auctions and on our own EE they are listed, bare bones, for $1700 or over.