My guess - not much. There are a lot of factors that go into how much a group of users is worth. Do they add a lot of value to your product? I would pay a lot for Woz, Bill Gates, Obama and The Pope to be active users of my product, because their membership would raise recognition of my brand. But just four random people? Worth a lot less. Four random people who are signing up and will forget about my service the next day? Even less than that.

An IPad costs about $500. If you gave one away for every 1000 people, you'd be paying $2 per person to try out your product. Assuming that your product is somewhat niche, you might have a signup rate of 1% (random guess) from a pool of the general population. This would make your cost per user $200.

It would probably be a better idea to imagine who your users will be and target this subgroup specifically. As an analogy, if I'm selling calcium pills to everyone I see on the street, I probably won't sell to many - no matter how good they are. But if I go find people who are afraid of osteoporosis, I will probably be able to sell to many of them.

I would suggest that you stop thinking in terms of absolute numbers of random people (especially if they're not paying) and think more about the value that they can bring to you (whether it is word of mouth marketing, content creation, advice, money or something else). For many products, random people are not the best choice for an initial user base.