Not sure what the question is. Global discounts have no limits on amounts (so, yes, you can buy 999 etchings... several times, if you want to and have enough rubies), and 999 x 7500 rubies is indeed 7492500 rubies.

Btw, if you're buying that much (or, really, just 10x1k etchings or more), there is a cheaper way to do it than through the global etchings discount.