Hi @aswin :
Let me answer your questions in turn.
Q1:I have a 10$ plan for power-apps, on top of that if I need to create a portal what plan should I choose?
Is your current license Per App Plan?If yes, you can create and access 1 custom portal.
Q2:Power-apps portal supports different authentication, if my organization use the app internally what kind of plan should I choose?
Access to custom portal requires related lincense.I think this link will help you a lot.(link)
User typeModelSKU namesUnit capacityPrice/monthChannelComments
| External user (authenticated) | Per login | Power Apps portals login capacity add-on | 100 logins | $200 | All | A login provides the authenticated user with access to a single portal for up to 24 hours |
| | | Power Apps portals login capacity add-on Tier 2 | 1000 logins | $1000 | All | |
| | | Power Apps portals login capacity add-on Tier 2 | 5000 logins | $3500 | CSP only | |
| External user (anonymous) | Per page view | Power Apps portals page view capacity add-on | 100,000 page views | $100 | All | |
| Internal user | Via license | Dynamics 365 (various) | n/a | n/a | n/a | Custom portal use rights are aligned with custom app use rights |
| | | Power Apps per app plan | | | | |
| | | Power Apps per user plan | | | |
Q3:Does power-apps portal allow free internal user login?
If the user has per app plan or per user plan, it can be accessed for free.
Q4: Can we create the login like OAuth rather than using the power-apps portal sign-in/signup form?
Yes.I think this link will help you a lot:
Configure OAuth2 provider settings for portals
Best regards,
Bof