Work backward from the business goal
Define what a useful enquiry or sale is worth to the business and what service capacity exists. Include production, agency, platform and landing-page costs where applicable. Avoid selecting a budget solely from a competitor claim or an unverified industry average.
Set a controlled learning plan
Choose a limited audience, offer, geography and test period the business can afford. Decide how spend will be capped and who can approve changes. Check that tracking and the landing page work before buying traffic.
Review quality, not just volume
Compare valid enquiries, qualification, response time and spend. Define a review threshold and what would cause the campaign to pause. Short tests and small samples can be misleading; no budget or agency can guarantee a particular result.