Transgender people serving in the military...
While it is true that certain medical conditions are deemed too much of a risk for military needs, I've seen nothing to prove that being transgendered poses such a risk.
While it is true that transgendered people have medical expenses, there are other medical conditions with higher maintained costs that are still allowed in the military. Also, there are many in the military who are still responsible for their own maintained medical care for various conditions that are allowed in the military. There is no proof that being transgendered is more of a strain on the military than some other allowed medical conditions.
I see no reason to prevent transgendered people from serving if they so desire.