$75 for a handgun, between $200-$400 for the rifles.
Let's say the courts find such a measure not to be in violation of the 5th Amendment (which is really unlikely). Let's then say that compensation is $500 per rifle.
Assuming we have 12 million of those in civilian hands (I've seen estimates between 8 and 20 million), that's six billion dollars. Assuming that half the firearms in civilian ownership are handguns and they offer a median amount of $300, that's 90 billion dollars.
Presented with such numbers, I wonder how many politicians would really feel so strongly about the idea.