It's not recommended to get implants before you start HRT for a variety of reasons, including the uneven growth thing you mentioned. Another reason is that without some breast development, there will not be enough skin to put implants behind.. And if you could find a surgeon that would do it before you had been on hormones a while, they would look FAKE.. Like incredibly fake, particularly if you are underweight like you say. You would also run the risk of your breasts ending up a big mess of stretch marks - which is not a good look. There is no skin stretching done before or during the procedure, the implants go in and it's up to your skin to adjust to accommodate them - this is why HRT first is kind of a requirement.
As for cost, it depends on the surgeon.. I've seen prices range from around $5000 to $8-9000. And there is zero Medicare coverage for this currently. As to the actual procedure, what gets done depends on a variety of factors, such as implant size and shape.
It's generally recommended that you be on hormones for at least a year or 2 before you think about breast implants.