You have two options:
1) Simple, buy Power BI Premium per capacity, where you can use e.g. P1 nodes each with 4+4core/25GB RAM with 5000USD/month/#P1. It is primary for cloud deployment, with additional benefit, option to use small on-prem server limited to 8core and only for PBIRS server (you cannot use SSAS or SSIS features).
2) Better (for large long-term on-prem deployment) is SQL Server EE with SA. There is little hard to say simple price, because there no monthly pricing, and there are two parts of price: once-charge and recurring , but basically, for first 3 years you will pay once 60000USD/2core, for next every year you will pay 5000USD/2core/year. You can use licence for physical cores or virtual cores, but better is physical cores, because you can share resources. You can use full set of SQL EE features (including SSAS and SSIS).
All prices are only aprox and for base level, real price (and discount) depends on your agreement. You also need some PBI Pro licences.
For large deployments, is best way to use both options, where you can scale as you need.
