How To Set Up Product Sales In Quantities ?

[kerintis]

Hellow fellas, got something in my mind I cant solve. I have products that sells in larger quantities and I want to be able to count stock. I can set it up over product attributes, when customer buys lot of 50 price increases xxx, but that's not helping, because its not deducting right amount from the stock. 

So I would like make if buying 1-50 price = xxx

if 50-100 price = xx and so on. Maybe there is a module for that ?