I suppose... if you don't pay any attention to their recent rulings.
Well, you're going to need to pay attention to all the rulings, not just the 2A related ones.
yes, recent SCOTUS rulings have favored 2A individual rights. But, SCOTUS has also ducked some very important challenges, such as those against AWB-like state legislation. Who knows if they would even hear a case challenging fees as unconstitutional?
SCOTUS has also issued a very problematic ruling on Obamacare that may actually play into this discussion. The majority upheld the Obamacare individual mandate by deciding it was a
tax, not a regulatory penalty. Congress has broad authority under its taxing power, as do states. If SCOTUS found this kind of gun fee to be a tax rather than a fee - or a penalty - then not only would they find it constitutional - they would effectively institutionalize the practice. And I think we would see another 5-4 decision in this kind of case ... unless the fee or tax was too high.
If such a "sin tax" were so stunningly high as to prevent individuals from enjoying their 2A rights (as withe the Mariana Islands, and potentially NJ), then I think SCOTUS would likely go the other direction and (at a minimum) find such a high tax to be an unconstitutional penalty. But it could signal tha a lower tax is still permissible. but much is too high? Is $200 too high? Well nobody has successfully challenged the $200 NFA tax yet. Would a 50% tax be permissible?
This is a big can of worms.
The poll tax proxy discussion is also problematic. SCOTUS never overturned poll taxes before Congress passed the 24th Amendment in 1964. The 24th Amendment gave SCOTUS the framework to find poll taxes at any level of government to be unconsitutional in 1966. But, before the 24th Amendment, SCOTUS did not find poll taxes to be unconstitutional.
2A says "shall not be infringed". A sales tax has not been found to be an infringment. Various fees and penalties have not been found to be an infringment. Even the $200 NFA tax has so far not been found to be an infringment (although in large part because this is limited to a special class of weapons). How high must a tax or fee be to infringe - to limit - an individual's ability to exercise their 2A rights? An across-the-board $500 fee would very much seem to be an infringment.
So where - between $0 and $1000 - is the line where a tax/fee becomes an infringement? SCOTUS is not likely to decide that specific number or percentage.