A 170 is a check flight to see that you are ready to take the test - I believe mandatory for the IR, most schools insist for the CPL too. Conducted by a senior instructor who is not you primary instructor, it is a more accurate judgement of how you will do under the pressure of a check flight with a differnet person in the right seat, like the conditions in your test.
I have never heard of a school charging a flight test fee for this (I can't see how they can, as it is not a test), and it can be part of the required training for that course. Mine certainly was for the IR - only wasn't for he CPL as I had been ill so required more hours for currency. I had also done plenty of "test profiles", and this was simply another one. Therefore it does not increase the cost to you unless you fail it. However this would suggest it has probably avoided you failing a skills test (a lot more exspensive) so you should then be grateful.