Most games in this space depend on virtual goods sales for their revenue. You know, "Get the sword of Unknowing and Libidinousness for $0.50" sorta stuff. A surprising number of players are quite happy to shell out their credit cards for such items.

For those who aren't willing to pay real cash, but still want them items, an option is to complete (free) affilate offers. Things like completing surveys, signing up to websites, downloading ringtones, etc. At the moment, those pay publishers quite well, and everyone wins.

We are using Offerpal network for ours, and when someone completes an offer to get a certain number of in-game points, we get the face value of those points in cash. CPM metrics are fairly useless for this sorta things, but eDAU (revenue per 1k daily active users) tend to be very good for games with well-designed transactions model.

This stuff is also fairly useful for anyone selling cheap things on line. If you're selling a thingybob for $5 apiece (website subscriptions, physical items, etc), you can capture cheapskates by having them complete offers. In which case you get an extra $5 from a customer who would've brought you no revenue otherwise.