I honestly thought you were being ironic at first. The pricing info is in their FAQ: https://slimfaq.com/faq (specifically at https://slimfaq.com/faq/24-getting-started/84-how-much-does-...)
Instead, all of their navigation is guiding me to the sign-up button as a next step. It's a product page for creating and maintaining FAQs, and they don't guide me to their own FAQ - that's not a good sign.
Also, $7/mo seems steep to me, but who knows? To me the FAQ system seems to involve an excess of clicking/navigating.
"SlimFAQ is free for up to 20 questions. If your FAQ requires more, you can sign up to our standard plan that comes with unlimited questions for just 7 $ per month (annual payment)."