Owning a gun is a choice. If you can't afford $.50/month to cover administrative fees then you likely can't afford the ammunition.
Really, this comes down to an argument for a registry - the fact that it would pay for itself.
Seriously - I can't believe any reasonable person would claim that $6/weapon is too stiff a price.
Heck, I'd even support tiered fees - single action hunting and target rifles could be quite a bit less than handguns and semiautomatic anythings. What's not to love about this idea?
EDIT:
those collectors with 100 weapons are likely already paying quite a lot more than $600/year to protect their investment - and this would be added insurance on top of it. Still not a compelling argument against.
That's $600 more added on top of $2000 property tax to keep a paid off house and $1200 for being alive (Obamacare mandated minimum insurance or fine for not having it). These are after-tax expenses that keep getting higher every year merely for being alive and owning things. Heaven forbid you lose your job and the government keeps adding these kinds of taxes.