I think your site will be difficult, because you only have 100k users a month, but high returning sessions. Showing the same ad to returning users typically doesn't advantage advertisers. Adsense would work better and give you a good RPM figure over time to charge prospective private advertisers, due to inventory size and showing ads based on user browsing history.
$2 per 1000 impressions is usually a good place to start, OR, implement an external ad company that allows real-time bidding on ad space. That way you let the advertiser determine the price and will quickly discover worth.