@Eric, when you look at the price list, you have to pay as soon as you add ads or affiliate links to your page, no matter how few visitors you get. As far as I know the costs are also not new. On a blog (don't remember where) I found a comment by an automattic employee that they didn't change the fee since the beginning of akismet. The only difference is the new account assistant which makes it pretty clear that you might have to pay for the service when you register your account.

My advise, there is really no need for Akismet. If your not if you have to pay, or you'r afraid you might have to pay for Akismet in the future, don't use the plugin. I and many other bloggers dumped Akismet, without getting flooded by spam. Try AntispamBee or NoSpamNX. Out of my own experience I can recommend both. Right now I'm using AntispamBee. It's under active development by a german wordpress dev, it's free for everybody, doesn't transmit personal data and blocks malicious behaviour so that most spam bots can't deliver their payload in the first place.