Three simple questions:
- Are you getting the hours you expected on the a/c?
- Are they looking after your a/c as you would expect them to?
- Is the rate you get a rate you agreed and are happy with?
If the answer to those questions is yes, then what they charge on top is really irrelevant to you.
If the answer to any of those questions is no, then go and have a conversation with them, and focus on those issues, not their rate.