Dependencies can be added for a specific product flavor, similar to how they can be added for specific build configurations.

For this example, assume that we have already defined two product flavors called free and paid (more on defining flavors here).

We can then add the AdMob dependency for the free flavor, and the Picasso library for the paid one like so:

android {
    ...

    productFlavors {
        free {
            applicationId "com.example.app.free"
            versionName "1.0-free"
        }
        paid {
            applicationId "com.example.app.paid"
            versionName "1.0-paid"
        }
    }
}

...
dependencies {
    ...
    // Add AdMob only for free flavor
    freeCompile 'com.android.support:appcompat-v7:23.1.1'
    freeCompile 'com.google.android.gms:play-services-ads:8.4.0'
    freeCompile 'com.android.support:support-v4:23.1.1'

    // Add picasso only for paid flavor
    paidCompile 'com.squareup.picasso:picasso:2.5.2'
} 
...