I don't think there's an exact answer I'm afraid, depends on the product, how much its going to sell for, and how easy it is to find good keywords that match your product (and which people are searching on!). But maybe £200-300.

What I will say- is that if a decent numberpeople are signing up with interest in your product, on the basis of a single page of html, with no release-date, screenshots or demos in sight, then its probably something they REALLY want!