OpenX doesn’t reveal its pricing. Though, it has been well-documented over the internet that they charge $1000 setup fee. As per your requirement; the number of Ad impressions, the minimum monthly fee is charged. For 1 million ad impressions, OpenX charges $17.50 as per the stats of 2015. This monthly fee goes up to $1200 for 50 million ad impressions.
This pricing was noticed back in 2015. Since OpenX is not an entry level Ad Serving platform, their pricing is subjected to be on the higher end.