The optimism of youth! Sorry Brendan there are no cheap 30 round mags anywhere to be had unless they are junk. This is because it is not legal to make new ones, so only what is left over on the shelves is available, and they are expensive.
Remember when you would go to a gun show, and if you couldn't find a gun to spend your money on, you would pick up at least a half dozen AR-15 30 round magazines to make it worth while spending the admission fee?
Heck, now I think a company could make darned good money by coming up with a vault specifically made for gun magazines!
By the way, what sort of penalties and paperwork are LEOs subjected to if they lose a post ban high capacity magazine these days?