It's good to put a vapor barrier they say now days but I have been a mason for 40 years. Back in the day we use to get the grade ready then rock it and then pour concrete. Now days they have modern technology, at lest that's what they call it, I think the verdict is still out.
They should had talk to you about what it was going to cost you up front, I know this because I am a contractor myself. I would had told my customer about what it would cost even if I could not had given them the final cost.