Bob- I was 14 back in '78. My older brother bought an AU-717 brand new back then- if I recall correctly, I think he paid about $350 for it, maybe $325. I can ask him as I'm pretty sure he still remembers. I believe he bought it at Crazy Eddie. Uncle Steve's on Canal St was another good place ( I got my Technics SL-D2 turntable there and my brother bought his Teac A-3440 open reel).
I dont know about there being huge discounts, but the prices were decent. I have a whole collection of hi fi mags from that time- there are some ads in the back with prices but not alot and those that are in there, are from states other than NY, I checked one mag and saw a price listing for the Pioneer Line with the most expensive SX-1980 selling for $788 (keeping in mind of course the value of the Dollar/Cost of Living in 1978).
I'm sure that newspapers from that time (such as Uncle Steve's ads in The Village Voice that I recall) listed models and prices but unfortunately I dont have any that I kept from back then. Perhaps if I'm in the library sometime and I'm inclined to do so, I can check the newspaper archives.
Regards, B/F.