The standard here is after 5 years on HRT and over 50 years of age.
Costs would depend on your insurance situation. Here, there is no charge to the patient.
Your doctor should be the one requesting it, but it would be up to you to schedule it with the lab. If your doctor doesn't think of it when you are due, you should remind them.
I have seen no statistics, so this is just my opinion... I think trans women would get less breast cancer than cis women of the same age. My genes may be 64 years old, but my breast tissue is only 2 years old. However, I wouldn't expect our rate to be as low as that of pubescent cis girls. There are those 64-year-old genes after all. SO, probably somewhere in between.