There are 1.3 million members of the US military.
There are between 1,320 and 6,630 trans members of the US military.
Total health care expenditures of the Department of Defense in 2014: 49.3 million.
Total health care expenditures of the Department of Defense on transgender related services: 2.4-8.4 million.
To give the analysis the most charitable interpretation of the numbers I will assume the maximum number of trans military members and that they incur the lowest possible cost.
Annual cost of non-trans military members: (49.3mil - 2.4mil) / (1.3mil - 6,630) = 36.26
Annual cost of trans military members: 2.4mil / 6,630 = 361.9
From this we can see that with the most charitable interpretation of the numbers, transgender service members cost 10 times more than regular service members.
Even though the total impact on military spending is incredibly slight due to the phenomenally low number of transgender service members, the business case for banning them is still there.
This policy change is an excellent distraction for an administration that recently failed to pass it's healthcare bill, and the post-modernist, hard-left ideologues are taking the bait very well indeed.