Marell Le Fou did some research on it and keeps spreadsheets of rare unit prices.
I still work on them and keep them available for anyone asking for a price or any information i have.
I have been looking for Warmaster every day in passing on eBay for 2 years so I can say with authority that the blisters have gone up in price, even for common armies, but it's still very variable.
I do the same and i have the same opinion.
On
old OOP stuff, prices stayed approx unchanged, with a serious diminution on very rare stuff. Hammerers went for more than 370$ for one unit two years ago, and several time more than 200$. Today, they sell for 70-80. Same for Savage orcs, for example.
On
recently OOP stuff, prices are gone mad. Dark Elves, Skavens, Brets and Lizardmen are going for crazy prices. Around 60-70$ per unit, NIB or not. Daemons where going on the same direction but they are more frequently seen on ebay, so i think their price is a little bit under the other four armies i have mentionned. Araby and Vampire Counts stuff we have not seen for weeks now (or very few units only), and the market is so versatile that i do not know for what they would go today. But i think it would be the same : prices around 70$ per blister.
I explain that this way : oldies interest collectors, while recent stuff interest collectors
and players. More interested people, so prices go up.
That is why you can buy Dark-Elves Corsairs for a lower price than Witches or Cold-One Riders...
One year ago, when GW stopped SG, I thought prices would not reach what they are today, or that prices would go up, but only for a small time. But i was wrong. It's something pleasant, in a certain way, that WM reach such a
post-mortem interest, but it's sad in the same time for it is too late to keep it alive.
Anyway, compared to 28mm prices, we are far far over classical prices... Mordheim prices do the same, staying high.
The day GW ceased SG, people rushed to buy everything available, even at high price, and it's still the case today.
As Stormwind says, even former common units go for crazy prices. Orcs and Gobs mainly. Undead, Empire, High-Elves and Dwarves remain lower. Chaos is between the two it seems. But it can change very quickly.
I try to keep tracks of who buy what, and i frequently see the same winners names again and again. I mean for any armies. So it would significate collectors still hold the top of the won items ? Or heavy players ? Hard to be sure. The only thing i'm sure of is that there are people making "now rare" armies and they are ready to pay a very very high price for them

The rule remain that with patience, you can have everything at low price... But it's harder than it was.