While it may indeed be the case that these fee's are "onerous and prohibitive for the storage and ‘administration’ of those firearms" it is common for the cost of storage to sometimes be charged back to the owner, such as with automobiles when dealing with a crime such as DWI, or death by vehicle. It is generally, at least locally, available to release to the owner, or lein holder, at either the end of the investigation, or the disposition whether release or auction/sale, or in the case of firearms, ordered destroyed, will be decided at the trial by the judge.
With automobiles, and other large items, it is understandable due to the size, and transportation issues, that they can not be handled b the investigating agency without outside help. The fee's and cost are subject to not only contract, but judicial review, as are other items in the contract.
With items the size of a firearm, and whose custody during an investigation must be strictly recorded, and controlled, this is the first I have heard of using an outside source for storage and disposition. I am unsure of the laws in MA in reference to these contracts between a storage facility, and law enforcement, but I am concerned about how the contract lays out not only the fee's, but also the exact way they notify the owner, and how the owner is allowed to have their property returned.