Say if the payout is $20 and you get 30 dollars worth of clicks and no sales should that be a sign that this campaign won't work out?
I might keep it going to $40, but much more than that and you should definitely call it quits or at least make major changes.
PPC optimization takes time..and even when you think you've figured things out, they can knock you down. In order to figure out what's going on, you should check your conversion funnel and figure where people are falling off..maybe the problem isnt with the traffic your getting, maybe it's a navigation problem or a bug on your site
Don't test using a set spend. Test using conversion rate. If you use $30, $40, whatever, if that gives you only 50 clicks, that's not a fair test, especially since many are saying a 1% conversion rate is typical. I would therefore wait until I get at least 200 clicks. That will be a fair test. But remember that conversion rates are not steady. You may sell 2 in the first 100 and none in the next 100 or vice versa. You have to look at the long-term rate over thousands of visitors to your site.