I think that your silver Catch would be +3 (+1 for availability) because bypassing the Catch would require the creature to take damage from a silver-based attack. And while anyone could pick up a silver necklace at the local mall, the necklace just isn't capable of causing damage unless it's hurled with magic (a rare combination). It's harder to come by heavy silver statues (to use as a club) or silver swords (which are difficult to make weapon-quality) or silver bullets.
I'm not terribly fond of the way the Catch math works -- that is, I don't like how it's a constant offset regardless of the size of the power it applies to (limited only by a minimum net cost of -1 refresh for powers). It would make more sense to me if the discount was scaled rather than constant. For example, a Catch that's fairly unlikely to come into play (the equivalent of a +1 or maybe +2 Catch) might give a 25% discount, which would still be worth +1 for Inhuman Toughness, but would be worth +3 for Mythic Toughness and Mythic Recovery taken together. Or a major weakness (the equivalent of a +4 Catch) might give a 75% discount, which could be worth up to +9 for both Mythic powers (but everyone who sees you runs immediately to the hardware store to pick up an assortment of iron tools).