If all you want to do is enable MFA, you don't necessarily need CA policies and Entra Premium. You have two "free" options:
- Enable Security defaults: https://learn.microsoft.com/en-us/entra/fundamentals/security-defaults
- Use per-user MFA: https://learn.microsoft.com/en-us/entra/identity/authentication/howto-mfa-userstates
As for Entra Premium pricing, you can check it here: https://www.microsoft.com/en-us/security/business/microsoft-entra-pricing