How to base price on quantity

i left this same question several weeks ago on your web site as a pre-sale question and received no reply, trying here just in case.
basically I need a way for users to select a quantity and then have price be calculated based on that selection. I dont really care how its done, using short code, action hook, filter, pro or free etc.
I just like to know if it can be done and see a sample since none of your samples have anyting like this.

Hoping for a reply.

Frank.