Do you have 100k active users per day or month?
I have 112k total downloads and about 24k daily active users. Each user generates about 8 interstitial ads per day, which equals to 192k+ ad impressions per day. My ecpm is about $1 to $1.90 and I make about $6,800 - $9000 per month.
I highly recommend switching to full screen/interstitial ads or video ads because the ecpm is much higher compared to banners. You could be making $30,000 to 36,000 per month!! if you switch to full screen ads and if each of your users is generating about 8 Ad impressions per day.
As far as I know,
Banners ecpm is about 0.15 - 0.90 cents per 1000 ad impressions
fullscreen/interstitial ecpm is about $1 - $3 cents per 1000 ad impressions
Video ecpm is about $3 - $6 per 1000 ad impressions ( Too bad there aren't any Video ad plugins for C2 :'(