The honest answer starts from what an enquiry is worth to you, not from a percentage of revenue somebody read in an article.
Ad budgets are usually set by what feels affordable, then judged by whether the number of leads felt good. Both halves of that are guesswork, and they make it impossible to tell a working campaign from a lucky month.
Start from the value of a customer
What does an average customer pay you, how often do they come back, and what share of enquiries become customers? Those three numbers give you what you can afford to pay for an enquiry. Everything else follows from that.
Budget for learning, separately
The first few weeks of any campaign buy data, not customers. Decide in advance what you are willing to spend to find out whether the channel works, and treat that money as research rather than judging it on returns.
Do not split a small budget four ways
A budget spread across search, social, display and video learns nothing anywhere. One channel, long enough to produce a verdict, is worth more than four simultaneous experiments that all end inconclusive.
Count enquiries, not clicks
Cost per click is a number the platform gives you for free. Cost per enquiry requires you to connect the ad to what happened afterwards, and it is the only one that answers the question you actually asked.
