Hi Tony.
I can't speak to the costs...you'll need to contact your Cisco reseller or Account Team for that, but I can tell you that LMS 2.6 has many bug fixes in it that you won't find in LMS 2.5.
On the other hand, LMS 3.0 is coming out very soon. If you're planning on an upgrade soon, I would look towards it.
HTH,
-Joe