Our CIO could speak to some of the cost and functionality parameters that defined our LMS migration to Sakai, but I can tell you that one of the critical decisions early on is: Are we going to host the LMS internally, or pay for hosting externally?
For us, external hosting of a Sakai instance has been cost-competitive with other LMS options, increases up-time, creates dialogue with people who are exposed to other schools' ideas and use cases, gives us a trusted training resource for Faculty and Staff, and even allows us to contribute to the open-source community responsible for Sakai. We partner with the Longsight Group, who re-distribute the modifications and corrections we need back to the community of Sakai users.
So are you going to host externally, or internally? How many users? Do you want the system to handle admin and staff communication, as well as learning content? How important are Portfolios to your work? Are you looking for a student-centred experience (Personal Learning Environment) or a true course-oriented solution?
Hope that helps.