The Kashan question certainly is an interesting one, especially the idea of actually using one as an price indicator! Although don't use this as your only indicator. You know the old saying about mathematicians: They can figure out seemingly impossible problems... but when it comes to simple mathematics, they'll spend just as much time, and make the same amount of mistakes as the average adult. The thing is, unless someone is extremely well versed with the markets, there are will be gaps in their expertise.
The following is completely out of speculation on my part, and is not written in stone whatsoever
There was a time in the 70's where Kashan's and Ferehan Sarouks were among some of the more favorable rugs out there in the market (silk rugs aside). As with the Persian Rug market (generally) after the 70's and into the early 80's, values of rugs started dropping of significantly. However, while some Persian Rugs seemed to regain ground such as the silk Heriz, silk Tabriz and other, select Kashans do seem to have been left behind, and stayed there to some extent. After a rush such as we had in the 70's, may be people realized Kashan's may be a little more common than expected? Today, high quality Kashan's still reach fairly high values, but I think there may be something else at stake here that's pressing them down.
Reds and Blues are always a staple Persian Rug color combinations. A Red and Blue Kashan was even the carpet of choice in The Big Lebowski for audiences to identify with. The thing is that Kashan Carpets, while very ornate, seem to have fairly replaceable elements on a commercial level. For example, one Kashan a friend of mine has is in the 300 KPSI range. However, in this particular rug he has (which is not true for all), very similar design effects and appearance could be created with a lower knot count. This is not to say the identical rug can be produced, but one very, very similar. Such would be the case of a Mashad rug on eBay, which I'm guessing by size could possibly be the case? These carpets were typically woven in this unusual 10x13 size, or maybe 9'9x12'9+ or something like this.
Anyway, what I'm getting at here is (I'll be waiting for Krow to step in on this one) using a Louis Vuitton handbag as an example: A fake one vs. a real one. Someone on the market for a $2,000 original handbag would not necessarily consider purchasing the reproduction for $100. Someone in the market for a $100 repro would probably not have the means, nor necessarily care to spend $2000 on such an item. But the fact is, the high price of the $2000 bag reinforces the market for replicas, and the replicas reinforce the market for the $100 repros. The difference is handbags are like cars. People buy them for status, they buy them for attention and all that other stuff. With Oriental Rugs, the reproduction of rugs does not reinforce the "original." It's not uncommon that a *unattended* customer with a $4,000 budget may only end up spending $800. While you could easily chalk this up to the fact that the buyer is making a safer purchase of an item which takes some know how to make an informed decision, I say people have perceived values. To many customers, two very similar rugs with very different price tags look the same. In regards to bringing guests in their homes, visitors probably won't ever know how much was spent on the rug if it was a lot or a little, so why be flashy about it? The thing is it seems like Kashan rug values have found a deep rut and stayed in them. The point I'm trying to make here is designs like Kashans which are copied over, and over, and over again, don't always appreciate monetarily from the production of similar replica items. In fact, 2 summers ago, I sold a 8x10 true Kashan from the 1920's (had a very slight color run and low but even pile) for $1000 less than the new Indian version! Maybe this is the time to buy them up?
Real Kashans (and I mean real Kashan's from the 1920's) are usually of great quality. Mohtasham, Manchester and Dabir Kashans are among the more sought after, as well as Souf Kashans (which I'm not particularly fond of) and others. These rugs have not taken a tremendous hit, but the more commercially available pieces of the 20's-40's have "depreciated." Keep in mind though, there are a lot of sellers who will refuse to sell these rugs for less than they paid. I guess this helps keep prices a little more stable.
Also important to add note to is that I believe these eBay Mashad rugs are not considered Kashan's, but rather of Kashan Design.
Here's an example of a newer and cheaper "Kashan"
Here's an example of an older (and slightly worn) authentic Kashan