There is a simple supply and demand argument that's not being mentioned. The number of men who seek to book CIS providers far out numbers those seeking trans providers, so much so that even though the supply of CIS providers is much greater, the ratio still doesn't come close.
There are a few other factors. First, there are layers of stigma. The stigma of booking any provider, CIS or trans, is a mental hurdle for many guys. That stigma and mental hurdle is even greater with trans providers. Many struggle to admit they are attracted to trans providers, and overcoming that and the stigma of booking an escort is a hurdle.
Another factor is some guys who find trans providers attractive and are open to them still prefer CIS women. However, at a lower price point they might rationalize it. If you charge the same as a CIS woman, you're not going to as easily attract guys who prefer CIS women. But if you come in below them, they might decide you're more worth it than the higher charging CIS women.
Also, there is a divide with trans providers that doesn't exist with CIS women. That is tops and bottoms. Tops, especially ones that can get hard, stay hard, and cum, are harder to find. All too often those who say they are versatile can't truly hold up. Therefore, they can command more money. When it comes to bottoms, many guys who seek them want them to be passable, aka look like a hot woman. The closer you are to this, the more you can charge. The further away you are, the less you can charge.
Now, technically, you can charge whatever you want. But if you want clients, you need to pay attention to what rate will get you clients. So, it's not always that simple. Mind you, I don't think $400 is all that high. And, as long as it's working for you, no reason to change it. But if it's not and you're mad about it, well getting mad isn't going to fix it.
FWIW, when I book a trans provider, I rarely book ones that are under $400. Most that I see in the cheaper category are not ones I find attractive. But I also won't go as high as I do for CIS women. There are too many good options for less for me to justify it.