WolfishMusings:
From a purely halachic standpoint I am not sure you’re right. The halacha you cited is only by a mitzvas aseh, and not by a lav. By the way the most one is required to spend is probably much less than 20%, see the Rosh and Tosafos to B.K. 9b. At any rate, everyone agrees that for a lav one is required to do whatever it takes as long as he is not putting himself in danger. One could possibly make the case, through an understanding of the difference on the d’oraysa level between a lav and an aseh with regard to this aspect, that an issur d’rabbanan (assuming there is an issur d’rabbanan on a woman not to dress in a provocative fashion) has the same status as a lav regarding this and not the status of an aseh.