App brain is best app promotion network currently available in market
it costs only 0.2$ per app install in high ecpm countries like us,uk,etc you can register on app brain by following this link
Advertise your Android app with AppBrain
App promotion networks are mobile ad networks or ad buying platforms that place ads across other mobile apps. They will usually charge on the basis of CPI (cost per install) or CPD (cost per download) and in some cases in a very sophisticated way, such as charging only for active users (DAU).