No fee for transfer to an heir. On death of an owner, the executor takes possession and BATF allows a "reasonable period of time" to apply for transfer to the legal heir. It is done on a Form 5 and there is no fee for a Form 5. The firearm may NOT be given to a dealer or anyone other than the executor to handle the transfer.
By the way, if the weapons are owned by a trust, they cannot be transferred to an heir since the trust is itself a legal entity and can survive the original owner. Ownership remains with the trust, so if you form a trust, be sure it's done properly.