Gunbroker final auction prices on small pistol primers are as bad or worse; $300-400/K is pretty common, and I've seen the equivalent of $500-600/K on auctions clearly marked for lots of 500.
It's true that some sellers set minimum bids or buy-it-now prices that high, but most set no or low minimums and it's the bidders who are driving the prices. Maybe GB represents the extreme response to limited local buying opportunities on a commodity that still has some availability nationwide. On my state's gun forums the price is hovering around $80-100/K, but recently ads offering SPP are fewer and are asking higher prices and ads wanting to buy or trade for SPP are up.