[Anaraxes]

I'll jump in hastily with -10 points were it a Disad, which is to say -10%. Points of comparison: reversed Magic Resistance (2/level). Cheaper than the example malus to reaction rolls (5% per -1), but the scope is more limited (pretty much enemy mages in combat, not everyone, and no one's going to mob you with torches, pitchforks, and hashtags because you cast a spell). Also reverse Resistant (were magic Very Common, +5 would be between 10 and 15 points; Common, 5-8 points, so 10 would be splitting the difference.)

Might want to phrase it as "after using your Magery", just to cover the case of the Magery 0 non-spellcaster that just wants to sense magic items, as well as other corner cases like vulnerability while enchanting S&S, and whatever else I've forgotten that's not "casting a spell").

IMO, -10 is punitive enough that I wouldn't take it for -20% to Magery. Probably even higher. -3 / -8 as "anti-Resistant" might sell better.