Start with a shared definition
Agree what makes an enquiry suitable: service, postcode, project scope and the ability to contact the homeowner. Report raw and qualified enquiries separately so changes in quality remain visible.
Work backwards from a won job
Consider gross profit per job, your desired contribution after acquisition and the proportion of qualified leads that become customers. Hypothetically, if you can afford £600 to acquire a customer and one in ten qualified leads becomes a customer, £60 is the corresponding qualified-lead allowance. This is an illustration, not a market benchmark.
Compare the full cost
Include media, management, creative and landing-page costs when reviewing acquisition. A low headline price for a lead tells you little about its exclusivity, intent or likelihood of converting.
Put it into practice
Review one recent group of enquiries with your sales team. Agree the quality criteria, identify the largest recurring mismatch and decide what to change before buying more traffic.