Certainly test with the cheaper clicks, and establish what kind of conversion rate you are getting. If you can convert one out of every 5 $20 clicks into a $500 sale you are doing great and should spend $20 on clicks all day long.
However, if you are spending $600 in clicks to get a single $500 sale, that's bad of course.
Also, consider the long term value of that click. Are you able to get people to opt in to a newsletter? From there, are you able to hit them with other offers down the road?
No matter how much you are paying for a click, you need to squeeze all the value out of the click you can.
Internet Marketing Consultant