How to get accurate average numbers?

Hi,

we have three shops and in each shop at the end of day we calculate avg price per item and avg purchase per customer. How do I set it up so lets say if i select data for a whole month it shows me the average price per item accurately by taking in consideration all turnover and all sold items? Because right now it is calculating  average number from column “avg price per item” which is mathematically incorrect.

thx Michal