Don't price this by the impression. If your advertisers are thinking CPM they'll rapidly start evaluating what this untargeted, run-of-network inventory is worth, which is likely close to zero. You won't get a $1.00 CPM if you price it by the impression. You'll be lucky to get a $0.20 CPM.
Instead, I'd try and sell a few sponsorships, Deck Network style, each sponsorship granting a share of the traffic for at least a month and for at least a four-figure sum each. The audience is looking at a Twitter widget, so your natural audience is Twitter-related businesses. Develop an unobtrusive text-based ad unit, something like the promos for Twitter clients that Twitter itself runs on twitter.com, and then pitch it to every Twitter-related business you can find.
Remember, high prices for a big share of a really relevant audience and do whatever you can to keep them from thinking about the inventory in terms of CPM, because five million ad impressions is not a hell of a lot when rationally priced. You need to do a lot of hand waving and sprinkle a lot of fairy dust ('reaching key influencers', etc.) to sell such a small amount of inventory for a meaningful amount.