Honestly, I don't like your rules.
Partly because I'm not sure I understand them. Partly because their damaging effect is mixed in with a Power-disabling effect. Partly because the process through which they inflict damage seems oddly convoluted. And partly because their costing method seems complicated and sloppy to me.
Yeah, it was kind of complicated and sloppy. My idea was to simulate various types of damage and power loss effects. A Power loss Bane might be running water for wizards and other magic-users. It doesn't really a wizard to be in running water (aside from the chance of catching a cold) but it prevents them from using their magic. A damaging Bane might be sunlight or holy water for vampires. Meanwhile Krytonite would be a Bane inflicting both damage and power loss.
I'd go with something like:[/quote]
BANE [+varies]
Description: There exists something that hurts you. Maybe you're a vampire that can't stand the sun or something.
Musts: If you have a Toughness Power, you must have a Catch or Limitation that prevents that Power from working against the effects of this Power.
Skills Affected: Endurance.
Effects:Bane. Pick a thing. Then pick a number X. You get a Refresh rebate according to a formula that takes into account X and how often you will face that thing.
Increased Damage. Any physical attack that makes use of your bane adds X to its weapon rating. An attack which uses it especially strongly add 2X to its weapon rating instead, while an attack which uses it especially weakly adds X/2 to its weapon rating instead.
Hostile Environment. When you are exposed to an environment which contains a small amount of your bane, you face an accuracy X weapon 0 physical attack against your Endurance each exchange. Environments which contain moderate amounts of your bane and large amounts of your bane inflict accuracy 2X and 3X attacks respectively. Being totally immersed in your bane inflicts an accuracy 4X attack.
[/quote]
Under Musts, I'd note that a Catch is essentially a Bane where X is 0. By definition all Catches are Banes. I'd also note that the modifiers for ease of acquisition and secrecy that are applied to Catches should also be available for Banes.
Finally, just as a general thing, I'd point out that Banes are one of the few, if any, Refresh cost break modifiers that should be available to Pure Mortals. A Bane could be used to simulate a person who has allergies to certain foods or insect venom. Or a Bane could represent a person with some other health issue that places them at greater risk. A person running into a burning, smoke-filled building to save a child is at risk. If that person has emphysema, cystic fibrosous or other lung disease, that risk is far greater.