Why Is This Coin So Expensive

What does DCAM mean?

The reason that coin is so expensive is because the 1999 Silver Prrof sets are the only way to get the silver quarters for that year. The 1999 sets are currently selling for $200-300.

Price and demand. Anything cost or is valued at what someone will pay for it. If someone says it is worth a thousand dollars and if someone buys it, then that is what it is worth.

Price and demand is kind of a glib no answer. The 1999 silver proof set has a mintage of about 10% less than the 2001 silver set and maybe 18% or so less than the 2000 silver set, yet the 1999 goes for an average of about $300, vs less than $150 for 2001 and about $30 to $35 for the 2000 silver set.
With mintages of over 800,000 and probably many many thousands in dealer hands, I doubt true shortages have occurred in the marketplace yet. With, say, a Lincoln 1909 S VDB, even assuming a 95% survival rate on the coins after 100 years, there are far more collectors than there are coins. I doubt this is really true of modern silver mint sets. Is the marginal value of a 1999 set really ten times that of a 2000 set? I can see the possibility of a lot of non serious collecting members of the public buying the sets the first year as something new and not following up on adding each new year, but would this be enough to generate a demand ten times less for the second year as the first? Even adjusting for the roughly 18% difference in mintages, this difference just seems unreasonable to me.
The Sacagawea proofs are another example I don't follow. The "key" is the 2001 S, yet 2004 S, 2006 S and 2007 S all had lower mintages, 2007 S by better than 30% - yet it on average costs about one fifth as much.
The numbers of these coins and sets sold with these huge price disparities preclude one from just saying "it's worth whatever someone will pay for it" as an explanation. It seems more herd behavior to me, if anything. One dealer starts calling it a key and raises his price, others see him get away with it and, viola, you have a new high priced key, even if it is not as scarce as some other lower or comparable mintage item in the same series.

Regardless of quantity minted of any coin, if no one wants them, the prices just drop, drop, drop. If you go to any book and examine the quantities of many coins you would see this. For example the 31D Mercury dime. 4th lowest mintage in the set. So what. Look at the prices. Many other examples such as the 1912S Liberty Head Nickel. 238,000 made and no really big prices. About double the mintage of the 09S VDB Lincoln yet many times less valuable. If everyone that collects coins wants certain ones, those are the valuable ones regardless of how many are made.
There are numerous examples of this in the coin hobby. For example if you go to the https://www.coppercoins.com web site you would find a double die cent in almost every year. Yet the only one people want is the 55 and the 72. Pop up with the others and so what.
No coins are basically a price and demand product.

If you are new to collecting and you were to come on this site you would see thread after thread about the infamous 1909s VDB cent (There I know nothing about US coins and yet was able to rattle that off without looking it up based on seeing threads on CCF!). Plus a few other "Keys"

You would soon get the impression that of all the cents there are this is the one that you need. The same is probably true in every series, that there are sought after and talked about "keys".

If you start collecting them you hear about certain ones and then you yourself feel you should have one. The demand is greater for these certain coins than others with similar mintages, or availability. Not as many people are after the others so the price does not reflect it. It's not so much supply and demand as it is hype and demand.

People like to be told what to do and what to buy. This is the basic issues with the crumbling economy. When I was a kid, all I had was a bike and a soccer ball to make me happy. What do kids need now day's ?

Most key dates are in ample supply for sale all over the web, making them hardly scarce. We buy, because we are told to buy, and I'm no exception.