Will, Parker Reproduction 28 gauges are awesome! I've owned 4 (2 now including one unfired one) so I have some experience here. Be aware that they are very light (around 5 1/2 lbs.) and some people have trouble shooting such light guns. I just shot 2 grouse today with mine and I love it (although I fell down and broke the wrist earlier this week...very big bummer, but it glued back together amazingly well!). Comparing them to AYAs is tough since AYA makes everything from cheap boxlocks to very nice, more expensive sidelocks. For $4500 though, I'd bet the Parker Reproduction would be as well made or better. Yes, $4500 is a good price for a single barrel set. Double triggers and beavertail forend (both very desirable to me) will raise the price if you can find them. Also they were mostly 14"-14 1/4" pull length which some people find short, but the combs were nice and high (usually about 1 3/8" and 2 1/4" drop). I hope that helps.
Joe Bernfeld