I was referring to the cost for the gaining provider. There should be no charge from the losing ISP e.g. cease charges unless the contract had some, or for example you are migrating away while within the minimum term of a contract, or other item such as a YouView box requires an extra payment.
Even if in a 30 day rolling contract, providers are still allowed to charge a reasonable notice period, which should have been defined in the contract when you signed up.