Bringing finances into a discussion of who's the best rapper was always weird.
Out of context yes.
But in this case, and Nas' as well for that matter, how much money they've made and how they made it (because of their craft) is a testament to their longevity which is certainly something to point up once you have a criteria in place that accounts for all the fundamental shyt.
I see people trying to separate "best" from "greatest" in the basketball stan wars.
Michael Jordan's greatness CANNOT just be confined to being the best basketball player, though he is. What qualifies him for best and GREATEST is his impact on the sport and that would include the mythos, the endorsements, the money he brought to the sport overall.
Jay and Nas both have stretched the window of financial success and longevity in ways nobody did before them and it's perfectly valid to point that up in relation to how GREAT they are. You might prefer this or that person more than Jay, you might think they are better at rappin but they aren't nearly as GREAT. There ain't a handful of rappers greater than Jigga. Pac, Jay and Nas are the only names in that mix.