I don't think the airlines even consider SARS when setting fares. Even If 50% of a given pop. at a destination had SARS I'm sure the airlines would keep fares high if the flights were packed.
Also, I've heard from 2 people who had planned on doing MRs to HKG and Taipei in late June/early Aug. These people moved their MRs up to May since fares had come down(at least on their routes) and they feared getting caught in some kind of quarantine later on. They say fares into and out of HKG have fallen a couple of hundred dollars.