You've yet to establish being transgender is a mental illness. As the clinical diagnostic criteria for gender dysphoria clearly states, the issue isn't with transgenderism in general but rather potential psychological afflictions (depression, stress, etc) as a result of which and that may manifest for any number of reasons.  Now substance abuse disorders are real, and the poor are more likely to suffer from these disorders. What your essentially arguing is that the poor could potentially be excluded or treated differently for the sole fact that they are more likely to have psychological issues, not that any given person does or will have such problems in the past present or future.