It's free for 1 Twitter account and up to 8 filters. After that, there's an annual payment of $20 for 3 accounts and 15 filters, or a $10 monthly for 10 accounts and 40 filters.

Thanks for the feedback about the copy. We're both developers which makes implementing features easy, but explaining how they work quite hard. We definitely need to take a look at the copy + add a demo of how things work. Just got to find a way to do it properly on a budget :)