Has anyone used OKTA with Appian?

No idea on the charges.
Yes, all of our internal users are being authenticated through Okta. We do have a couple of external users and some of our admin accounts that use what we call the workaround. Rather then going to company.appiancloud.com/.../tempo they use company.appiancloud.com/.../login.jsp. In this way you can have users that do not have to authenticate through Okta but use the internal (Appian) authentication.
Task notifications go directly to the task if the user has already logged into Okta. Otherwise they are redirected to login to Okta and then go to their task.