+3, as noted by others, is probably a good baseline, assuming +2 availability & +1 researchability - however, were I GMing, I'd consider going up to +4. Here's why.
RAW, you get a +2 from having your toughness powers only protect against X, and a +0 for the default of protecting against everything except X.
When X is, say, fire, this is easy to adjudicate: +2 for your powers only protect against fire, vs, +0 for your powers protect against everything except fire.
But consider: which sounds like the better description here:
Your toughness powers only protect against non-living attacks.
Your toughness powers protect against everything except living attacks.
They both sound pretty reasonable, really. So I'd be tempted to award an extra +1 for, in essence, having a really broad catch. It's not quite broad enough to get the +2 for only protecting against a limited range of things... but it's far broader than a normal +0 type catch, which could still be something very narrow like fire or cold or iron.