You might be about on with your numbers.
Unfortunately, with the receiver and DVD player you have to find someone who wants something of that era. A $100 Panasonic BD player will deliver today's tech, which blows away DVD, for almost nothing more. So, it has to be someone with an extensive DVD collection who wants really good DVD upconversion, but doesn't want to buy a current generation Oppo player. Not exactly applicable to most buyers.
Likewise with the Integra receiver.
I have an old RX-Z1 from Yamaha, which I figured was going for about $100 on eBay, but it seems to be selling for closer to $200+ which is a far cry from the $2,800 is cost when it was new, but indicates that it still carries some value and people want the excellent quality it delivers. So, you may see $100-$200 for the Integra.
The speakers retain the most value for sure. Speaker technology is not like receiver or component technology. If they are in good shape, then they should retain about half of their original MSRP and it seems like that would be fair. At 1/4 of their MSRP someone is getting a really good deal IMO.