One key part of formulating your business model is understanding how your customers will pay. Sometimes your paying customers are users of the product; sometimes the "customers" are third parties such as advertisers or partners. Besides deciding on who will pay, you also must research them to understand:
How much they will pay. How much money do they have? How much money do they make? If confronted with a real buying scenario, will they pull the trigger?
How they will pay. Do they have access to on-line payment systems (e.g. PayPal)? Do they require CFO sign-off? Do they have and use credit cards? What do they use to pay for similar products?