At first it may be once a month as the doctor tries to figure out how rapidly your endocrine system reacts to the hormones. Mine is consistent with the 3 month rule, but she is doing what she wants to do.
Keep in mind that the tests may change. For example, what I listed was my first test. My second test was only for prolactin, total testosterone, estradiol, and CMP. The thyroid test was excluded since it tested normal. Since my prolactin, and metabolic levels were very consistent, and my testosterone remains very effectively suppressed, she is only going to test for estradiol and liver panel. In other words, my first test was $433, my second test will probably be around $360, and my 3rd test will probably be less than $200.
I may point out that I'm under 30 (slightly) with no medical history at a healthy weight. My doctor is also very understanding about trans issues and knows how money can be with us. If you are more of a medical risk, or you get a doctor that couldn't give less of a crap about your personal life, you may find yourself paying for more tests.
Also, that $400 a month doesn't include the doctors visit... my endo costs an extra $410 a visit. This stuff is not cheap at all.