That's the point people are missing. Also the flipside which is if it costs some bugger $1000 to mine a coin, he ain't gonna sell it for $100 either.
But others will, so, who cares what that bugger's cost was. Market determines price, cost is irrelevant.
However, once market has set a price, the cost will tend to go somewhere close but still below that price. So, price causes difficulty.
But, this also confuses many people into thinking that difficulty causes price, and markets being what they are, that thinking can actually cause the price to rise, when people think it should rise because of the difficulty rising, even if difficulty increase is caused by other reasons like change in mining tech. Therefore, bubbles.
There is an another mechanism, although a slow one, that regulates this positive feedback loop, see here:
https://bitcointalk.org/index.php?topic=414657.0