Take the value of one customer to you over the time they stay. Multiply bythe share of enquiries that become customers. That gives you what one enquiry is worth. Divide the monthly cost you are considering by that number, and you have the only figure that matters: how many extra enquiries a month this has to produce to break even.
Now compare that against what is realistically available. If breaking even needs forty extra enquiries a month and the entire search demand for what you sell is a few hundred people a month, the arithmetic has answered you and no supplier can argue with it. If it needs two, the decision is easy and the remaining question is only whether this particular supplier can produce them.
The number most people are missing is the last one, and it is the cheapest to get. Real search volume for what you sell comes out of advertising platforms, it is bought once rather than subscribed to, and the method for getting it honestly is a morning of work. Doing that before signing anything is the single highest return hour in this entire process.
One adjustment worth making, because it did not exist five years ago. On a growing share of queries the answer now appears above the results and nobody clicks anything, and that lands hardest on short definitional questions. Forecast those low. Comparative, situational and anything with a purchase behind it are largely untouched, which is where the arithmetic above still holds.