[Closi]

It will depend on what you are looking to get out of advertising - immediate sales or engagement/awareness.

If you are an eCommerce business selling a widget then you can easily get a cost per conversion, and then it's pretty easy to optimise your spend to maximise profits (i.e. If I spend $30 on ads, I can sell a product I buy for $40 for $90 making $20). I've seen businesses with this revenue model who will spend the equivalent of c.30-40% of revenue to get the sales due to the high margins often involved.

Your conversion will depend a lot on your product, target market, the ad etc (if you are selling a business rolodex your CPC might be better on linkedin, if you are selling a selfie-stick your CPC might be better on insta etc).

The effectiveness of advertising for engagement/brand awareness is much softer, and the spend will be much more discretionary as it is harder to track. It's these ads which are more likely to fall away IMO.