Hi there @MattK,
I think you'll find this post helpful.
In a nutshell:
- You can embed a public app (which doesn't require user authentication) in your website.
- This wouldn't incur in any cost are the users seeing the app wouldn't be retool users under your account
- I'm not really sure about the iframe, although looking at other posts (e.g this one) I think it may be possible.
Just bear in mind that when you make an app public, anyone with the link will be able to see it, which may be a risk for your data.