515, much like the concept of cloud, MPS is really more of a business model than a technical description of what you can and can't do, and I certainly encourage a holistic approach to how you look at MPS, even up to including document management and workflow opportunities.
To Old Glory's point, I was brought up in a world of blending rates, but dual rates is certainly fine as well. One thing I've seen be very successful is implementing rules based software tools to help you and your customer guide volumes where you both want them to go. Price certainly plays into this, but employees don't always keep their eye on the company's bottom line like you and your decision maker might.
I hope that helps, and good selling out there!