I've never used Clicksor, so I don't know if they pay per click or per impression. If the latter, then their rate is horrible: ~$0.20 CPM. Stop using them immediately if this is the case. If they pay per click, then it's kind of unfair to measure their service with impressions rather than clicks, although a low CPM usually means low CTR (click-through rate), which in turn means that their ads are not very good in that they aren't appealing enough or aren't delivered to the right target audience.