I feel part of the problem with people not giving Lex his financial due is where a lot of the Lex's real-world perception problems lie:
Richard Donner and Gene Hackman (and Bryan Singer and Kevin Spacey).
Thanks to Donner and his accused protege Bryan Singer, too many people still see Lex Luthor as a cartoonish buffoon obsessed with land, and not a cunning, murderous self-made billionaire in his own right.
To me, Luthor should the legally richest man in the world (and in the running for illegally richest man in the world alongside Ras al Ghul and Vandal Savage and their immortal ilk).
Take the fortunes of Bill Gates, Steve Jobs and Mark Zuckerberg at their respective peaks, and Lex Luthor's legal wealth should still be more than theirs combined.