The Grendelkin has an magic immunity (worded as "standard mortal magic or perhaps just fire", but it's still of 'equivalent specificness' as far as the rules go) pair with a +0 baseline Catch. So a magic-specific Stacked Catch is always priced at +5, regardless of baseline Catch. I'm assuming that all of them are priced exactly as the example used for Stacked Catches: +2 for specific, +2 because not-magic is common, and +1 because research uncovers it.
That said, OW and YS don't really seem to use the same rules all the time, so comparing examples isn't always a perfect solution. For example, by my read, the standard Fae Catch should probably be +4: +2 because iron is commonly available, and +2 because it's a trivial task to find info on the Catch by Googling it (assuming you didn't already know it). My best guess is that the second value is treated as +1 based on it not always being obvious that the individual is in fact Fae to begin with, but that's just an attempt at rationalizing it.
