An auto optimized ad or an Auto oCPM ads in Facebook/Google/instagram is indeed the best way to scale campaigns. However, do keep the following points in mind, else you will end up burning much of your $100:
The top bid management platforms are: Kenshoo, DoubleClick Search, and Marin. Obviously, you can run your account just through AdWords itself, but if you want a platform with lots of features to potentially help your bidding, reporting, etc.
No matter the size of the account, I could handle the management and drive better performance than an automated system. It just won’t know the ins and outs and history of an account in order to manage things properly. Not to go all “man vs. machine” but I have yet to encounter an automated bidding solution which outperforms what I or other experts in the field could have done.