Satoshi thought and planned for it.
If the bitcoin price falls below the cost of mining, some miners will exit. It will temporarily cause block discovery to lengthen as the existing miners striuggle - but after 2400 blocks, the difficulty will adjust down. That means it's easier to mine. (The difficulty is directly linked to the cost of mining as you need many more miners to win a block when the difficulty is high).
Basically you'll get iterations of this till the cost of mining falls below the bitcoin price. If the bitcoin price fell to $1 I expect you'll be able to mine them with your laptop as the professional mining farms will have long given up.
Bravo! Nice answer and I like it the way you explained it. Yes it's true that with decreased prices of the bitcoin the tension on the blocks will reduced down as miners will not be able to cover up the earnings and then keep the miner live.
In addition to this there will be one major indirect factor that will play role in demotivating the miners to exit the field and that is reduced number of investors and also existing investors will pull back from the bitcoin world as the prices drop to couple of dollars. This way GPUiners will benefit lot. You can confirm your own transaction by using your computer power by including the txid. That would be cool.