I'd change that very slightly. It's not their willingness to pay, but their cold, hard cash that signals how much they value the product. Many, many people will say they're willing to pay for a product before you build it, and they really are, but won't actually pay for it when it comes time to sign up because they either no longer have the need, or the product isn't actually a good fit, or they can't afford it, etc. You can't really believe any signal except the money going into your bank.