Insert a column while not changing formula

I have read through countless forums and tried several times to get this fixed by reading others problems/solutions. This may also be something pretty basic to most of you, but to me its been very frustrating.

I keep track of all of my grocery costs in an spreadsheet. It has the product in column "A", the size in column "B", average cost in column "C", and the cost per unit in column "D". I insert a new date and the prices in column "E" and shift older dates to the right. I need a way to shift the column to the right and have the average price reflect all of the dates, instead of shifting the column numbers. Aditionally I need to be able to copy this formula down the rows in the event of adding a new product.

I have experimented with absolutes ($E$2), and tried figuring out INDIRECT and INDEX functions, all of which has failed me.

Any ideas? I have attached the file for assistance....THANK YOU!