The prices mentioned are based on basic priceplan. The more You spend, the cheaper price per install You get. We have 4 priceplans, check them in "My profile" section in your dashboard.
Note that You pay only for real installs, not for position or rank.
For example: you want to increase your app's rank for the keyword bmw drift. Right now your app is on position 52 for chosen keyword. To increase your app position to the TOP-3 for this keyword you will need approximately 500 - 800 installs daily.