For ad based/impression revenue, using the Google Firefox/Toolbar referral fee is a good rule of thumb.
For Google, each user that installs the F.F/Google Toolbar, and hence will be exposed to ads each time they search the web is worth $1 Dollar, or at least that's how much they are willing to pay for each user having Google as their default search engine.
So I would say $1 Dollar per signed-up user, +/- fudge factor if you really have or haven't additional leverage on your users.
This makes no sense for an anti-spam service. They aren't "signing up", they are "opting out" of all future emails from whatever they are using the email for...so this user will probably come back, but if you spam them even once, you've lost them for life. They aren't the same as "signed up" users for a toolbar or installations of software.
This is a pure ad play, in its current form, and has revenues from that source. There might be ways to integrate this with other anti-spam, anti-malware, privacy, whatever, features to build a product suite that is worth more than the sum of its parts, but right now it's an ad-supported model.
For Google, each user that installs the F.F/Google Toolbar, and hence will be exposed to ads each time they search the web is worth $1 Dollar, or at least that's how much they are willing to pay for each user having Google as their default search engine.
So I would say $1 Dollar per signed-up user, +/- fudge factor if you really have or haven't additional leverage on your users.