If you kill him with melee, sometimes you won't even notice the cash he drops since it'll be added to your bank, and people sometimes miss the fact that they just earned a bit more money.
Base Genki kills will earn you $100,000. If you have the "All Pedestrians drop 2x cash", then you can up to it $200,000, and by combining all the other upgrades, you can get it as high as $300,000 for killing him. During my most recent playthrough, I only saw him once.
Also, sometimes he'll spawn with a rocket launcher, which will be dropped if you kill him. This is especially awesome early game if you haven't unlocked it yet.