Which is the cheapest and efficient way of collating data across different locations for a POS system?

Creating customer profile based on the average items bought or the average weekly sales to collect data and add to profile.

a hard drive in a store room or small office in a small work station dedicated to save all data of the accounting system the pos is running this will keep records off all sales and creates a customer data base to view.

You should Run the Software by which u may get all the Sales of products. e.g in MS Dynamics u sholud run P-Jobs to collect the Data from all POS