Update: Please note that this was originally written for Spam Karma v.1 and therefore some elements (licensing especially) no longer apply to SK2. Same spirit overall though.
OK, so before I go any further, let me make things as clear as I can:
Spam Karma is and will always be absolutely free. Free as in beer, free as in love.
So free, actually, that you could practically take it, change twenty lines and start selling it as yours. Except your own karma would probably shrivel as a result.
Anyway, the gist of it is that you are under no obligation, legal or moral, to pay anything for its use. In fact, you are entirely welcome and encouraged to use it for free. Though a supportive e-mail or comment is always nice too.
Now that we got that part out of the way: