Solution seems to be to track your conversions carefully. If you can justify your cost per lead or sale based upon your resulting ROI, then you're winning. Yes, of course there's noise in the data (bogus clicks, unqualified people who you don't really want to target but mistakenly do, technical glitches like your website not responding for an hour, etc) - but if you consistently measure your conversions, what's important is the trend in your ROI, not each bit of noise.
Hope that helps.