Your earnings rate (CPM) can vary according to several factors, possibly including your niche, the source of the traffic, and some other things. Once you account for the percentage of pageviews that go to HubPages, you can expect an average of somewhere between $2 to $3 CPM.
Let's say you estimate on the high end. 300 page views per month * 30 days = 9000 page views per month. 9000 page views per month * $3 per 1000 views = $27 per month. All things considered, you'd probably earn somewhere in the neighborhood of $15 to $25, depending on how close you got to those high end estimates.