The number of users you may or may not get per month is dependent on product, message, and media, and promotion (budget) involved. Start slow and think big.

You are going to have to give a good bribe to the customers..whatever that is. It must be better than a free newsletter. But information products are the best due to the low cost involved and the high perceived value.

Best of Luck,
Mike
From the Trenches to the Towers Marketing
I will be glad to help as my time permits.