He is attacking fees applied to the purchase or registration of firearms. Even not knowing about the Murdock case, I would agree that imposing a tax or fee to engage in a constitutionally-protected right is not permissible. However, I would take it a step further: I think that -- on the same basis -- all permitting schemes for carrying firearms are unconstitutional.
On the basis of Murdock, though, a state might then argue that, "Okay, we'll eliminate the fee -- but you still need a permit to carry a gun." And I respectfully submit that even that is (or should be) unconstitutional. We don't need permits to worship. We don't need permits to write letters to the editor, or to our elected representatives. The RKBA was one of the rights that the Framers were most concerned about preserving, so the notion that today we need a permission slip from the nanny state before we are graciously allowed to engage in the right is unacceptable.