Rare? More like 'one in a million' I'd say. I spent about a year looking for AGs a while back. The cheapest I ever found was a pair of Duos in Germany for around £4k, which sold in hours. I'd actually phoned the guy to ask if he'd hold them for a small deposit while I sorted out getting there in a large enough car, but they'd already gone.The OP shouldn't give up on Avantgarde quite yet. A couple of years ago I found a pair of 2006 AG Duos advertised for just £2200. That sort of bargain is rare and I spotted it minutes after its listing and bought them next day. I was specifically looking for Duos and set up alerts when new ads were posted. I suggested Unos as they are less costly and not a lot short of Duos in sound quality - comparing earlier generation with 2 separate horn tubes.
If you find a pair for £2k and they're legit, then that's serious bargain territory. The point though is that if you wait for that sort of deal, you may have a very, very long wait indeed.
In the meantime, you'd not go far wrong with the Snells in the classifieds for £500. Preferably before I make the pointless decision to end up with even more stuff that I don't need