SELECTEDVALUE in calculated column

Hi,

I am looking to do something in a table, but I need to use a selectedvalue function to achieve the best results. Basically I have a quantity value, and am looking to apply a percent change to those values, whether it be an increase or decrease. Is there a way to get around the fact that I cannot do a selectedvalue in a column.

I have attached some images of wqhat I am working with.

In the end I am trying to apply a percent change to the quantity, before then taking that new quantity value and multiplying it by an entered number based on month and "Cut" of meat. 

Here is a picture of the price table too.

Thanks!