It all depends on the niche in which you work. For example, if the cost of a lead is $30, and he buys for $500, it is usually quite good, but if the cost of the lead is more expensive than the purchase, that the client makes, you should pay attention to advertising (creatives, texts, target audience). If the advertising indicators are within the normal range, it is worth doing an audit of the site or another page where the person goes from the advertisement. Use testing and prediction to find what you can improve to increase conversions. It is also worth considering the customer's LTV.