When Do You Let Your Horse Die?

From your guys experience, when do you shut off a keyword/URL? When its past the value of the lead? 2x the value 3x? For example if I'm promoting a $4/lead offer and target siteoffer1.com has alread gone through $4 worth of clicks do you shut it off? Wait for $8? I know the law of averages states you should let it run, but when is it time to stop spending on a dead horse?

Thanks in advance for your insights..

I'm no PPC expert, but I'm pretty sure that $4 worth of leads definitely isn't enough to see if a keyword is profitable.

That's a hard call to make on a $4 lead. For some insight when I promote a $30 offer I will run a $60 campaign. But I will track everything - this is key. By the time I spend $60 I will have a list of targets that have data and I can shave off the ones that didn't convert. That is when I can determine if the campaign can be profitable or not.