When you're working with someone you should have a basic understanding of their business model (offer, deal size, margin, etc). You also need to understand their sales process (cycle length, method, close rate, etc). Ideally you can do this during onboarding a client. How much a lead is "worth" essentially comes down to profit and close rate. With all that being said - it also pays to have some industry knowledge in your clients domain - At least to the extent that you have clarity on how qualified your leads are.