Someone at Retool can correct me if I’m wrong but from what I understood, each of your portal users has to sign up as retool end-users meaning you’d pay for them like you would pay for any other end-user. Since Retool Portals only works on Business plans or upwards, the pricing for portal users would start at $15/month/user.
[Edit by @kbn : see below reply clarifying that Embed users are still counted as end users] However, if I understand it right, users of embedded apps don’t count as retool end users since you have to manager authentication instead of retool. So if you’re willing to deal with a bit more complexity you might be able to save a few bucks.