Emily Greer, co-founder of Flash game portal Kongregate, remains
skeptical as to just how significant advertising revenue could become —
at least for the average developer. She points out that for the vast
majority of web-based game developers, sponsorships alone comprise
50-70 percent of any income they earn; these days, a good game can net
between $1,000 and $3,000 in sponsorships and a great game — something
like Desktop Tower Defense
— can take in as much as $20,000. By contrast, only one or two
developers within the Kongregate community earn $1,000-$2,000 a month
from ads; some five to 10 of them earn roughly $500 while between 40
and 50 take in a mere $100 each month. And that’s out of a community of
about 2,500 developers. She does, however, admit that those numbers are
on the rise.
Link. So it's the same story we already knew about monetizing casual games. Performance based stuff doesn't really work, and sponsorships are the best source of revenue for game developers. These don't really scale elegantly though.