IANAL, but it would seem to me that when one company purchases a contract for goods or services from another, they are also purchasing along with it all of the terms and conditions of that contract, none the least of which would be the negotiated costs...
...that is unless you agreed to an amendment to the contract at the time that the service switched...
What kind of correspondence took place between you and the new host when this transition took place? Is it possible that perhaps they pulled a fast one on you by sending some fine print your way that may have gone overlooked?