Power BI online integrate into SharePoint on-prem. extranet Architecture

Power BI Embedded Online Licencing as of 22 Nov 2017:
Basically, there are 2 parts to licencing PowerBI online.
1.> You licence per the number of pages you render per hour.  You need to have the Power BI Embedded licences on infrastructure to serve up a certain number of requests per hour. So work out your peak number of page request per hour and licence for the appropriate plan.  The table below shows the Power BI Embedded plan you'll need to subscribe to:

Plan	Virtual CPUs	RAM (GB)	Max Request per hour
A1	1	3	300
A2	2	5	600
A3	4	10	1200
A4	8	25	2400
A5	16	50	4800
A6	32	100	9600

Note:  I believe the plan's can be scaled up or down instantly without display and pausing a service stops the Power BI embedded costs.  If you run over the Max requests per hour I believe the Power BI PaaS will still serve up page/reports but you will get an extra bill for the additional reports.

2.> You'll also need to purchase at least 1 Power BI Pro licence, that is used for: administration, content publishing, and development..
3.> As of time of writing (Nov 2017) the Microsoft Power BI Gateway does not offer High Availability (HA), but I'm sure it is coming soon.
4.>  A single account is used to connect to each source and RLS security has to be applied at the source (SQL SSAS), user table mapping is required.

Also see:
http://blog.sharepointsite.co.uk/2017/10/power-bi-on-prem-extranet-information.html
https://docs.microsoft.com/en-us/power-bi/developer/embedded-row-level-security