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The Radeon R9 280X is a nice card for its MSRP of $300, at least assuming that you don't mind the relatively poor energy efficiency. New Egg has some in stock. The cheapest is $400.
Wait, what? $100 over MSRP is the cheapest, and prices go up from there? The card is effectively a rebranded Radeon HD 7970, which is just shy of two years old. And it's a card that AMD is still actively manufacturing and selling. The Radeon R9 290 and 290X are also far above MSRP, and that's even for the reference cards that you definitely don't want for gaming purposes. So what is going on here?
The answer is Bitcoin, Litecoin, and other cryptocurrencies. Modern Radeon cards have some sort of bit-shifting instruction that modern GeForce cards lack. For games, this basically doesn't matter; GPU code in games deals almost exclusively with floating point data, and bit-shifting floating point data will do very strange things. Bit-shifting can be useful as a high-performance way to multiply or divide integers by powers of two, so CPUs get some modest out of it in a lot of programs, and it's also very trivial to implement in hardware. (Think of it as multiplying by powers of 10 when doing arithmetic by hand: tacking some zeroes onto the end is much faster than going through 0 x 3 = 0 a bunch of times.)
But the hash functions as used by Bitcoin, Litecoin, and other such cryptocurrencies are likely to involve a ton of bit-shifting. That Radeon cards are massively faster at it than GeForce cards means that Radeon cards are massively superior at mining such currencies. Efficient bitcoin mining has largely moved to FPGAs and custom ASICs, but that hasn't yet happened for Litecoin and some other cryptocurrencies, leaving high end Radeon cards as the most effecient way to mine them.
So what happens if you can buy a $300 video card and expect to make $20 per day doing Litecoin mining with it? You buy as many as you can, and they sell out at $300. When vendors like New Egg realize that they sell out immediately at $300, they raise prices until the cards stop selling out immediately.
So why did this only happen recently? Because prices on Bitcoins and other such cryptocurrencies have spiked upwards. Bitcoins first broke $250 in November, and went over $1200 earlier this month. Other, newer digital currencies have seen similar spikes. That makes buying up high end Radeon cards and using them for mining currencies highly profitable, at least if you believe that prices will stay that high.
Which they won't, of course. It's hard to imagine a future in which dozens of currencies are used interchangeably even in a very small geographical area. I expect cryptocurrencies to never expand beyond a tiny niche, and likely disappear entirely. But even if they're here to stay, the world would most likely converge on one, rather than having people mine dozens of them, some of which may not be as secure as people think.
The reason prices stay so high is that if you believe every card you buy is going to be highly profitable, you buy a lot of them. Whereas a gamer rarely has much use for more than one card and almost never has much use for more than four, currency mining scales well to arbitrarily many cards. So people will buy dozens or hundreds at a time if they can. If AMD could bring twice as many cards to market, those might all sell out, too. And that's hard now that AMD has to divert a ton of production to Xbox Ones and PS4s.
So I don't know when prices will return to sanity, which we could loosely define as "not much greater than MSRP". At the latest, Radeon GPUs aren't anywhere near optimal chips for currency mining. FPGAs would probably be much better, and custom ASICs certainly so. Those will arrive for other currencies, just as they have for bitcoins, and that will dampen interest in using video cards for coin mining. But that could take a while.