That is their motto agency fee collected up front and lady works for tips that is where the upsell comes in.
I wouldn't call it naive I would say you just had no idea of how this hobby works great you found TER.
Providers here on ter have a flat rate for each hour.Whatever the hour rate is that is what you pay.
There is no expectation of a tip when dealing with a "true independent provider"versus an agency who acts as if the ladies are independent when they are not.Which is what you get when you mess with the cards on the strip and yellow page ads.
Stick with independent ladies here that have their rates posted on ter profile and websites.
And if you want to go with an agency check out 100proof agency listed here.
Welcome to vegas and ter:

Head over to the newbie board. You can find it on the general boards drop down. Read back a few pages and also check out the TER self help center near the life preserver

...and how much did she ask for the upsell?  

Sorry this happened to you, bro. None of the professional TER ladies that you find on this board would ever ask for an additional fee or unless it was for an upgrade for services not automatically included in her standard profile fee.

We all get scammed at some point in our lives, maybe even more than once..
Don't worry about what it cost ya, it could have been way worse and next time you now have resources to use.
Xoxo