In cost plus pricing the discount would, for the most part, be very small because the price was tied to the cost. Typically higher volumes involve some efficiencies of scale that might lower associated costs and be passed on to the customer. There might also be a small additional discount based on the idea of rewarding the customer that was buying more.
When you go beyond cost plus this kind of discounting gets much more aggressive. The discount is no longer tied to any cost savings but instead takes into account the concept of diminishing marginal utility (the idea that each subsequent unit of a product provides less benefit to the user than the one previous) for the purposes of revenue management.