I saw another thread that was closed around this topic. From a usability perspective, the way you have to find out what it costs for upgrades is horrible. One point called out in the other comment thread is that you can't find "upgrade" from your dashboard unless you have already created a free account. I am deciding whether I want to use WordPress or not. Forcing me to create a free blog so I can get information to help me decide if I want to create a premium blog is a waste of my time. And forcing me to dig down into your support comments logs to figure that out is an even worse waste of my time.
Suggestion: Add content to your site that displays upgrade costs that is accessible without having first created a free blog. (or) Add content to your existing pages on upgrades that states that until you create a free blog, you can not find, anywhere, what it would cost.