We recently were given a small database (65K) of opt-in email addresses by a friend of ours who collects them for third parties. My boss wants to repay the favor in some way, but we have no idea how much email addresses are worth. I've always gotten my subscribers the old fashioned way, never by buying them. I realize that these kinds of lists are really only noteworthy if they're relevant to the consumer's interests in your industry and if they convert properly, but in general how much are email addresses worth?
