First let me thank everyone who participated in the discussion about add-on pricing. There were many great thoughts shared and it helped me make my decision on add-on pricing. What I have decided is to stick closely to what EllisLab already does with ExpressionEngine. The exception will be MSM sites. Essentially I will sell one license per installation of ExpressionEngine. So if you run a single domain or 100 domains off of a single EE install you will only require a single license from me. I will stick closely to the volume discount rates that EllisLab has setup which is outlined on this page.
I realized the multi-domain license didn't seem like the best idea after everyone's comments. It would add more overhead than it's worth when it comes to time. Also many people mentioned confusing people and from a business perspective I don't want to confuse anyone. So there will be a single license and a single price-point. I also will steer away from the developer's license since I will implement volume discounts. This means you will need to register as a member on the site in order to purchase and track your purchases. I think people will be willing to register an account if it means volume discounting.
Now that I've made the decision, I just need to build the payment system to support this. That is, unless someone else builds and sells me one first :)