The optimal mix of Power BI Pro and Power BI Premium licensing in terms of total cost will vary based on the volume of users and the composition of these users between read-only consumers of content versus Self-Service BI users. In relatively small deployments, such as 200 total users, a Power BI Pro license can be assigned to each user regardless of self-service usage and Power BI Premium capacity can be avoided. Be advised, however, that, as per the following Power BI Premium features section, there are other benefits to licensing Power BI Premium capacity that may be necessary for certain deployments, such as larger datasets or more frequent data refreshes.

If an organization consists of 700 total users with 600 read-only users and 100 self-service users (content creators), it's more cost effective to assign Power BI Pro licenses to the 100 self-service users and to provision Power BI Premium capacity to support the other 600 users. Likewise, for a larger organization with 5,000 total users and 4,000 self-service users, the most cost-effective licensing option is to assign Power Pro licenses to the 4,000 self-service users and to license Power BI Premium for the remaining 1,000 users. 

See Chapter 12, Administering Power BI for an Organization, and Chapter 13, Scaling with Power BI Premium and SSAS, for additional details on aligning Power BI licenses and resources with the needs of Power BI deployments.