How much do you need to spend?
Most budgets get set by picking a number that feels affordable and hoping. This works the other way round: start from the customers you need and let the arithmetic tell you what that costs.
Free, no sign-up, about 2 minutes. It will also tell you if the plan is too small to work — which is the answer more often than people expect.
Paying customers, not enquiries.
Check this against last quarter rather than from memory. It is usually lower.
Clicks that become enquiries. 2–5% is typical for lead generation.
From your account, or Keyword Planner if you have not started.
Gross revenue from one customer.
The chain, and where it breaks
Customers → leads → clicks → spend. Each step divides by a rate, which means each rate you get wrong multiplies the error at the end.
The lead-to-customer rate is the one to check hardest. It is usually quoted from memory, and memory over-weights the deals that closed. Take last quarter, count the enquiries, count the customers, and use that.
The conversion rate is the one you can most easily change. Two to five percent is typical for lead generation; if you are below that, the cheapest budget reduction available is fixing the landing page rather than buying fewer clicks.
What the model deliberately will not do
It will not tell you paid search is a good idea. It returns arithmetic, and arithmetic is happy to produce a viable-looking budget for a business whose customers do not search for what it sells. Paid search has exactly one precondition — people search for the thing before they buy it. If they do not, no budget fixes that.