Now there’s a good question.

Thinking this through for a few minutes, my instinct is that Luthor is probably richer than Bruce for the very reason that he’s a villain. He is more willing to do things in the course of doing business that are illegal, or that hurt people’s lives, or that cut corners in order to increase his bottom line, whereas Bruce is not. So even if Bruce started out with more, my expectation would be that Lex would lap him at some point.

Of course, that’s all mitigated depending on what Lex’s status quo happens to be in whichever iteration of the DCU we’re talking about. If Lex has been convicted of crimes, he may have had assets seized or liened in order to compensate the victims of his crimes for their damages, and so forth. Plus, for all that Bruce needs to buy a ton of batarangs and constantly re-equip Batmobiles, Lex’s equipment and weapons get destroyed on a regular basis, so his monthly expenses are probably a lot greater as well.

Regardless, Tony Stark can buy and sell them.