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I think the position of your ads also contribute to the value you earn.
I have read that ads display near top of page has more $ value compare with one near the bottom.
300x250 Image Only ad units (looking for videos)
Change it to text and image.
There probably aren't that many video ads at the moment. If you have image only, the level of competition will be very low and you'll be virtually giving the space away. By allowing text, your video providers will have to increase their bids to get them seen, and if they've invested in video then they will probably be prepared to increase their bids.