This can truly can be an exercise in frustration. I was recently looking at CX prices for HK-SIN economy roundtrip and it went from HK$2,100 to HK$2,700 back down to HK$2,000 within 1 week (for the exact same flights on the exact same dates). I guess the lesson is if you see a price you are happy with, just book it. Or wait and risk it.