Write a small billing tool that can ingest two XLS files and output a CSV or XLS

I need a simple billing tool written that will do the below. I would prefer it written in something that can be easily updated such as Access but any solution that works is acceptable. It must run on Windows. - Once a month, I get a XLS pricelist from a vendor with less than 1000 subscription products in it, with a cost and RRP price. Each product has a unique SKU - Once a month, I also get a CSV billing output, which has a list of customers, subscriptions, qty and totals based on my cost price - The tool will merge the data as per below and output it as XLS or CSV The tool needs to be able to do the following once a month via a manual process: - ingest each months price list, and save it as the purchase and sell price for any subscriptions with a purchase date in that month (the purchase date is listed in every billing file, so it does not need to be recorded) - ingest and examine the "start date" of each months recon file, look up the appropriate buy price and sell price, and collate to an output that includes the cost AND sell price for each line item, and a total for each customer - provide a separate lookup table where we can manually put in an internal customer ID, PO for each customer that is output - generate an internal SKU from the vendor SKU (it's a simple modificaiton of the vendor SKU) - Ideally we should be able to record an over-ride Sell price for any customer, save that in the DB, and the tool reports the cost, RRP and Actual sell price for every customer Please provide estimates of initial build cost. Ongoing modifications will probably be required as the vendor changes file format from time to time.