People with gender dysphoria have been enlisting for years, just as gay/lesbian/bisexual people enlisted for years before the policy was changed in 2010 (and before even Don't Ask Don't Tell). There are about fifteen thousand transgender troops, counting both active duty and reservists, according to a study last year.
The numbers (at least the ones I've seen cited today) on the cost of transgender-specific health care come from a Rand Corporation study commissioned by the DoD, so they're not being pulled out of a hat.
Anyways, the medical costs are a fig leaf. Trump's announcement wasn't that transition costs for troops wouldn't be covered by the government, he's saying transgender people are banned from serving. With no explanation of what this means for a transgender member of the service who's currently serving overseas.
This is about making sure a particular group of people remains stigmatized and treated as less than the rest of the population. The groups lobbying for this policy aren't deficit hawks who are looking for ways to cut waste from the DoD budget, they're largely Christian conservatives who resent the growing acceptance of transgender persons in the US. Sometimes it really is that simple.
