I have tried this out, with no module, but by adding an extra text field to the product type (Don't make it, so it must be required).

I was also using commerce feeds and feeds tamper(For multiple taxonomy terms to work) to upload the products from a CSV file.

In the CSV file put an extra column for the RRP or the price, the original price, the price is being reduced from. Make sure the title for that column is going to match to the extra field you put in the product type. In that column (Unlike the actual real price column, where the actual discount price will go). Put in the dollar sign, in front of the price and the decimal in, like you would normally write it. For instance $45.00 as the it is just going into a plain text field and is not going to be processed. The reason, I say to put the dollar sign in the actual CSV file on the RRP price column, is that if any products are not discounted, you would just leave them blank, in the CSV file for the product they match, then you will not have a stupid dollar sign showing on non discounted products, with no price in front of it. I also used CSS to target the class that surrounds the RRP price to put a strike through it and change the colour of the price to red text-decoration: line-through; (perhaps I put a custom class in the view, for the wrapper of the RRP price, I can't remember).

Obviously if you wanted to just discount things by a percentage amount across ranges of multiple products then, my way, would require you to fill in extra prices in column of CSV file manually. Which may not be ideal for some situations. But, I hope what I have written, is helpful anyhow.