This has been noted elsewhere, but I didn’t see any post here. This is a pretty annoying bug because Aimed Shot is part of the standard MM rotation and this ends up costing a lot of extra mana over the course of a fight.
Master Marksman (5/5):
Increases your critical strike chance by 5%, and reduces the Mana cost of your Steady Shot, Aimed Shot, and Chimera Shot by 25%.
When all 5 talent points are spent on this talent, the mana cost of Aimed Shot is unaffected – not reduced at all. At level 80, before the talent is chosen, Aimed Shot costs 403 mana. With 4/5 points spent in the talent, Aimed Shot is reduced to 322 mana cost. Once the 5th point is spent, the reduction disappears and it once again costs 403 mana.