I actually like to add an intermediary step between the landing page and the e-mail form: a "Create an account/Buy now for $X/month/year" button. When people click on it, they get a "Sorry, we're not done yet, but give us your email and you will be notified once we release" + the email form.

Not only do you know people are potentially willing to pay X dollars for your product, you can also do a multivariate test on different amounts or pricing schemes to see if they convert differently, giving you some rough idea how much your idea is worth to them. I always add a "Make an account for FREE" as well, as a baseline.