To echo what several people have said, definately talk in person. But having said that, your child is asking you to support them, which it seems you are doing, but also you want them not to make an irreversable mistake. The thing about going ahead with HRT while not only having had very few counselling appointments, but openly rejecting them, is worrysome to me. Why are they being rejected? There could be many things here, your child could be correct, they are TS and don't need someone to tell them that, possible, even probable. They could also just have found a bad counseller, some don't even believe in or acknowledge TS. Or your child could also not be truely TS, or not yet ready for this step, and the counseller picked up on it, and your child resents the counseller for it.
Also, the part about not dressing very often has me curious. It is true that dressing really has little to do with TG/TS, but someone who does little to identify with the target gender but still identifies as TS does raise some flags. In my case, I am officially diagnosed, which is helpful in dealing with all of the other professionals in the process, but I also rarely dress, not because I don't want to, but because I have other obstacles to deal with. I do try to express my femininity in subtle ways as much as possible, and this helps. Perhaps this is what your child is doing at this point.
The sperm banking thing I see in two ways. First, yes it is a sensible and mature precaution, but it also strikes me as a bit of "have your cake and eat it too" thinking. Yes, your child deserves to have children of their own, but being TS does involve giving up some of the things of the birth gender. I am of very mixed feeling about that whole aspect, as I am sure are many here.
As for the money, $500 is a drop in the bucket when it comes to TS stuff. Most of the MTFs have spent many times that on something as simple as hair removal, I know I have. When you get to things like FFS, Breast augs, and of course SRS, then you are talking real money. Your child needs to consider this, life ain't cheap, and being a TS is really expensive. If they are worrying about something like this money wise, what is going to happen when they get to the really expensive stuff, are they going to expect you to pay for that too? Before they go very much further down the road, they need to develop some sort of financial plan.

Personally, I think that your child does owe you the simple courtesy of talking to you about what they want the money for and the consequences of their actions. It does not seem to me that you are trying to stop them, just making sure that they are not making rash decisions, like a parent should. I am having enough problems dealing with my own transition, dealing with someone elses cannot be easy either. Good luck to everyone involved and let us know what happens......Stephanie