Just been through this pain with Microsoft and whichever way you try to wriggle they have thought of it and you pay. We looked at all permutations of core + cals.

We have a large 64 core 2008 R2 cluster server running an enterprise server license which is reasonably priced with software assurance. Go to SQL 2012 - this license becomes invalid as the maximum number of cores is 20 for a transition. We would have to throw away our old license and spend big bucks £60,000 + on a new licenses. It's cheaper for us to buy new hardware and transition our server license to 20 cores..