The real cost of Google advertising is not what you pay your agency, but the cost of the actual advertising itself. This is because you can waste thousands of dollars in advertising to the wrong audiences with the wrong keywords, match types, bid strategy, improper conversion tracking and mismatched ad copy and landing pages. Like maintaining a car, you will get by without fluid changes and proper tire pressure for awhile, but that will often result in far more trouble and poor performance than if you would have paid a certified mechanic in the first place.