Excel 2007 : Maintain Formula while Sorting Cells

I've tried looking for this, but I can't seem to find the solution, maybe I'm not searching for the right terms, I don't know.

So, this is what I'm trying to do:
I have an Excel Workbook, with several sheets.

The first list is a Cost of Supplies sheet, with lists of the supplies I've bought, how much they cost, and where I got them.

The next is an Item Catalog with the item name, description and quantity on hand. I was trying to make a column called "Cost to Make" which used a formula to to add the prices of the supplies from the Cost of Supplies sheet, tell me how I spent to make the Item. The formula looks like this: ='Cost of Supplies'!C6+'Cost of Supplies'!C7+'Cost of Supplies'!F30

On the Cost of Supplies sheet, I use the Sort function to sort the supplies alphabetically. When I went back to the Item Catalog sheet, my formula remained the same and therefore was using the wrong cells and giving me the wrong price.

Is there a way I can sort of have the formula update so that it follows the cells if they are moved during a sort and thus maintains the correct cells and data?

There's probably an obvious answer to this, but I'm fairly new to Excel.