If you think about it as a diminishing returns curve, it becomes something that can be understood and explored. Of course, the sales people will try to get you to expand quickly and your most efficient spend will come first - after your inflection point, your incremental return will decrease.

However, fighting that pull is the optimization opportunities enabled only at scale. If I run 1 ad, I know how that performs. If I run 1000 ads, I know which ad performs best.

So what I used to tell people to do was to slowly incremente their budget and measure performance at each step. It's kinda stupid, but it worked.