How do you best find out a providers rates if they are not posted on the website?

It should be accurate if the reviews are recent. If she's not reviewed, perhaps you should see someone else.

GOOGLE her email address and phone number...
You might find other ads on other websites, which list her rates.

& Is she reviewed, ask other reviewers, who have reviewed her.

In an email I express my desire to schedule some time with her...I include verification info and a little about myself...then I say something like " I never bring up the subject of rates in an appt request , but if your site mentions rates I fear I missed it.  I know what it says on  your TER profile but I do not want to assume that is accurate or current.  So you might have to give me a heads up...thxs".  

You can call and ask, don't be shy.
Or an e-mail will work

I did that to a certain person and she totally cut me off now.

If not, you'll have to ask.

I generally have not had issues when calling getting the rates based on time of stay. Just don't discuss services during the call, that will never end well.