apply a discount to product in Commerce
I use the Commerce Sale Price module with great success on my website. It is easy to configure. Just install and enable then you can set the discounts on the product pages.

Update: I think you could use Commerce Coupon to achieve your results. The coupons can be automatically applied based on a role. Sorry I did not understand your question fully before. Also, I have not used this module but the description appears to do what you want.