Some altcoins got hit hard when exchanges allowed the rates to go below 1 satoshi... This means there essentially is no hard low limit for them. There is also some people who intentionally exxagerate the rate drop for some altcoins by mining them and selling for less than mining costs. I can't say how long they can sustain mining for loss, but that will for sure hurt reputation of involved exchanges and give clear signal for other traders to stay away from those exchanges. When there is so many sell orders at sub-satoshi rates, it will take a lot more than 1 BTC worth of buy orders before the price can climb back to usual levels around 40 satoshis and up.