I don't think it's about money really, it's a question of where they'll be more successful. Like Toxic just said, most of these guys aren't at a UFC level and the ones that are wouldn't be top talent. It loops back to the age old question, would you rather be a big fish in a small pond, or a small fish in a big pond? I know what I would pick.
Edit: not to mention of course that the thought of OneFC making enough money to pay their fighters more than the UFC is almost laughable, and that's coming from a huge One fan.
Compared to UFC's good draws certainly not, but as long as they are lower/mid tier fighters (UFC level, but top OneFC level) they probably can match the pay. As they don't have US name value, the UFC won't pay them that much anyway, and those $4k+$4k(w) what the UFC pays their lower tier fighters without name value is something OneFC should be able to outmatch.