If you only wanted this for a particular item like item ID 36, then you would use something like:
TotalPrice = ([Price] * [Quantity]) - (([ID] == "36")?floor([Quantity]/6):0)
And then maybe add a few calculations so you can display them on the page where you need to:
DiscountAmount = (([ID] == "36")?floor([Quantity]/6):0)
The calculations are actually very easy and just about any discount is possible by using simple algebra.