If I believe the wiki or the thumbnails, upgrading a mex to a given tier cost the exact same price as building it from scratch to the same tier.
(T2 -> T3 == T3)
This would imply it is more interesting to reclaim, then build a mex rather than just upgrade it.
However, this guide: http://supcomfaguide.uw.hu/chapter1.html implies a cost of 3600 and my own test games implies 3450.
(replay number:3474101, name Dawngame, map Summer Duel, only player is suka).
Is there an official thread about that? Any info?