Retail TFS does not come with 5-CALs. It has a EULA exception allowing up to 5 users without CALs. The primary difference is that CALs can be used to access multiple TFS instances. A EULA exception cannot. In other words, buying two TFS retail licenses does NOT give me rights for 10-users on one instance of TFS. It gives me rights to two instances with 5-users each. To add more than 5 users, you must have CALs for all additional users.
Each TFS deployment allows access from 5 users without CALs. Once you get beyond 5 users, each user must have a CAL. That's a serious hit once you go from 5 users to 6 users. Microsoft seriously needs to straighten out their licensing madness.
Edit: I read that incorrectly. Once you go beyond 5, you need a CAL for each additional user.
So, it's USD399 (upgrade) for 5 users, and USD499 each for the remain 5 users. So it's USD2900 to upgrade TFS itself. Then you need to upgrade each copy of VS.