The cost of transferring a .com depends on the policies of the losing and gaining providers. Ordinarily, you'll only need to pay your new provider a fee (usually around £10) to cover the cost of a registrar transfer.
In some cases though, you'll find that the provider you're transferring away from (e.g. Namesco) imposes a charge for unlocking your domain and allowing it to be transferred - a highly questionable and unfair practice that impinges on your right as a registrant to transfer to another registrar.