Start with the sale. What does it cost to deliver the product or service? What is a customer worth on the first purchase? What are they worth over the full relationship? How much margin is left after the work is done? Until those numbers are clear, an ad budget is only a guess.
Suppose a sale brings in $1,000. If delivery costs $600 and the business needs $250 of the remaining margin, the campaign has no more than $150 to acquire that customer. That is the ceiling. The target should be lower, because not every cost appears neatly in the ad account.
Now work backward. If one in five qualified leads becomes a customer, the business can afford up to $30 per qualified lead. If the landing page, sales process, or follow-up changes that close rate, the acceptable lead cost changes with it. Media performance cannot be read separately from what happens after the click.
This is why we ask about COGS, customer value, margin, offer, and follow-up before we talk about spend. Sometimes the numbers support an aggressive campaign. Sometimes they show that the offer needs work first. Both answers are useful. The job of the read is to know which answer is true before money moves.