I don't think my income's ever increased past the standard $2395 per 70ish seconds from devil's drink fighting -> sleep in slum? Though I suppose it increases a bit further if you can add more restrictions, until you can like 3shot Lin Lin with them I don't see it as being worth it.
$500 indeed isn't that much, but I'm not sure why you're implying that this is something that you only get later on, since even before you get strong, double-wielding hammers is adequate for good rates.