I work in ads for the airline space, I'm on the engineering side so I can't give great financial numbers offhand, but we usually do trial runs on just subsets of routes an airline offers, and for every customer so far the increased revenue on those routes heavily outweighed the new ad spend. Of course you can make bad ads, just like you're saying. We have departments dedicated to that though, I myself couldn't tell you too much about making a good ad that a few SEO blog posts wouldn't cover better.
But it can be formulaic to a degree. If you're an airline, give us some of your metrics and we can get a pretty good estimate of your ad performance at different levels. Got a SaaS product, video game, new snack? We're useless, but there's likely some company of analysts out there who have you covered.
I'm sure it's extremely variable by market though. Air travel has huuuge volume and lots of customers from every tech-proficiency level. I often get YouTube ads for B2B SaaS services and other extremely targeted business things, and I wonder who is signing a 20mm contract that started from a YouTube ad?? Maybe some other commenter will tell me it works though.
Edit: Actually, I signed up for Robert C Martin's (Uncle Bob) new software training video site recently because his youtube ads were so silly. It helps that I knew his name long before though, and I've yet to pay for one of the non-free ones.