Again, about Kidd, all we saw was simple metal attraction and repulsion. That is a basic ability. More advanced would be metal bending. Even more advanced would be overcoming magnetic resistance in non-magnetic metals, and then even more so with non-magnetic materials in general. If Kidd can bend metal, his bounty would be justified in my opinion. If he can move nonmagnetic things with magnetism, he'd be on par with a Yonkou, seriously. If he can't bend metal, he doesn't deserve that bounty.
Thinking about it, why is Moria so powerful, but someone like Mr. 5 is not? That's because Mr. 5 was satisfied with the basic ability to make things explode, while Moria took simple shadow stealing and made it into zombie making, shadow manipulation, strength enhancement, zombie manipulation, shape shifting and shadow switching. Crocodile said that DF powers can develop and grow, although he himself couldn't go much further than manual water absorption, disassociation, and forming sand weapons. Luffy amazed us by turning mere stretching (and electrical immunity) to power amplification, Gear 2, and Gear 3rd.