Email her and ask her where you can find her rates. Rates should be on her website. Btw, is this girl highly reviewed with many reviews?

...the information is on their profile here. If they don't have their rates posted anywhere else then you have to use the amount listed on their profile. You could ask them if the rate on their TER profile is correct. Don't see the ladies you mentioned if they don't have their rates posted where you can easily find them. Don't see them if they don't have reviews. This is the advice I give based on you being a newbie. Just taking the "going rate" and hoping that works is not the way to do this.  

Recommendations: Get VIP membership (if you don't already). Go to your TER regional ad board. Find some ladies that interest you. Check their TER profile. Read their reviews. Check their web site. Pick one. Set a date.  Have fun.

Well you are saying she doesn't want to discuss rates over the phone.  So just send a message and say "I saw your ad on xyz, what is your donation?"

Ask a direct question, get a direct answer.   You don't have to play charades with this type of question.  

I've never had a real issue doing it that way, unless the provider is an idiot or something, which we all run into from time to time.  

What you don't ever do is blatantly discuss over phone or by email any types of sexual acts.   But you can discuss "donations" all you want.  Conspiring to donate money to people for no apparent reason is still legal as far as I know.