I have no reason to think that 50GB of ROM couldn't be manufactured for around the $1 price point, I simply don't know (in small runs it's probably much more, which may be a problem).
I have no concept of the economics of ROM, and suspect (perhaps naively) that you don't either. But for flash memory, I see 32GB for $0.76 here (https://www.alibaba.com/product-detail/Bulk-micro-size-sd-card-32gb_60489692297.html). The economics of solid state are dramatically different than even a short time ago, and that's simply what I can find while quickly searching.
Definitely won't be as cheap as a mastered BluRay, but getting close enough that it's not an insane prospect to have the other advantages of solid state (no need to "install" games that still require disk swapping to play, and 500GB is pretty small when you point out that games use 10s of GB each.