flavorDimensions 'brand'
productFlavors {

    brand1 {
        dimension 'brand'
        ...
        buildConfigField "String", "BASE_URL", ""http://brand.one.api/""
        ...
    }

    brand2 {
        dimension 'brand'
        ...
        buildConfigField "String", "BASE_URL", ""http://brand.two.api/""
        ...
    }
}

android.applicationVariants.all { variant ->
    if (variant.getName() ==~ /brand1Api[0-9]{2}Staging.*/) {
        variant.buildConfigField "String", "BASE_URL", ""http://brand.one.staging.api//""
    } else if(variant.getName() ==~ /brand1Api[0-9]{2}Production.*/){
        variant.buildConfigField "String", "BASE_URL", ""http://brand.one.production.api/""
    } else if(variant.getName() ==~ /brand2Api[0-9]{2}Staging.*/) {
        variant.buildConfigField "String", "BASE_URL", ""http://brand.two.staging.api//""
    } else if(variant.getName() ==~ /brand2Api[0-9]{2}Production.*/){
        variant.buildConfigField "String", "BASE_URL", ""http://brand.two.production.api/""
    } else {
        throw new Exception("Unexpected variant name: " + variant.getName())
    }
}