Exporting these two reports and combining the data in a spreadsheet does not account for existing inventory, shrinkage, FIFO vs LIFO, etc...so the best you can do is export the data to Excel to calculate an average sales price and average purchase price (assuming a purchase for the SKU occurred in the time period for the two reports). 

I am shocked this basic functionality is not available. Please let me know if there is something I'm not understanding.