The spreadsheet will handle 13 different prices per SKU - one for the previous year inventory and one for each month. If a SKU is purchased for different prices in a single month, the average of those prices will be used.
If you want a more exact COGS for that SKU, change the date for that PO entry to a month that you did not purchase that item, or to a month with the same price. Just don't forget that this will affect your PO purchase data for that month.