The pricing is listed on the website, but it's about 10-20% of that. You would pay around 0.50 per 1000 emails. Depending on how often your users sign in and how long you keep them logged in for the the percentage may be lower or higher.
Not offering SAML/SSO/OIDC etc simplifies the problem a lot allowing me to offer it cheaply. The largest cost is sending the e-mails reliably, for which I am wrapping Amazon SES.