Obviously, retail pricing on these units from Service Merchandise. Stereo stores, retail, not warehouses might charge a bit more and then give discounts based on a system purchase. I have a receipt from such a store for an SA-9500 that was purchased alone at MSRP.
Pioneer was generally available for 70% of retail at the discount warehouses and this for the SX-1280 was $950 List, $665 warehouse pricing and $749 Service Merch Price. Warehouse price is calculated.
Let's to the SX-1050 and the SX-5580 Black/Silver® version available in the PX/BX system. Retail was $750, 70% is $525, easily warehouse purchased for this, District Sound Warehouse had them for $478 in their catalog in 1977. I have a receipt for the purchase of the SX-5580 I have from the PX and the selling price was...$353.10 no sh!t, just 350 bucks for a Pioneer SX-5580, a great looking multi-voltage version of the SX-1050, that was a deal. No wonder so many GIs hauled home big rigs back then.
@jt01, the SX-680 had a list price of $300, you can pick a price point from the above data and be happy with the adjusted for inflation price of that receiver since yours was well bought, nicely repaired and sounds great.