Why? Nikki, that question is asked, and often screamed, thousands of time a night. There are many answers, and way too many none answers out there. When you first dressed as a girl did you do so because you were dressed that way, you were experimenting and allowed to do so by permissive parents, or was something you experimented with in secret? Those are just three questions that come to me. I'm actually not asking for an answer, just presenting them to you to consider.
The question of how much, depends on what for? If you can get a wardrobe, a home and a job as Nikki, the cost in money is quite low (depending on the wardrobe, or course.

) Possible insurance coverage effects the total cost greatly. That considered, I've heard estimates of from $20,000 to $100,000, perhaps wildly out of date estimates by now, but posdsibly somewhat close. That all depends on how much change you need to achieve a comfort level - probably a comfort plateau. One major cost is time. It takes at least two or three years on a fast-track basis. Only in dreams do we go to bed male and wake female. Rats!!
The first thing is a cousellor, one qualified in gender identity disorders. It is a logical first step, one that hopefully gets you some answers, but also the first step in a legal and medical process, regardless of how far it might lead.
So please, work on that, and remember your friends here. We do not have all your answers, but together, we have many that may help you.
SusanKG