I've been using FV Antispam on my multi-site install and it's impressively effective--enough so that I also installed it on a client's site. It puts anything from bots right into the trash. It's meant to work together with something like Akismet that can identify human-created spam, but it's been quite sufficient on its own, because there are so few spam comments posted manually that moderating them isn't terribly hard labor.

On the other hand, for a business site, the $5 or $10/month that Akismet asks isn't that much. Popular sites make more than that from AdSense in a day. So it's not unreasonable for them to ask you to pay if you are making money from your site.