Ok, I want Power BI but I do not know which option?

That was the question I got, and there are actually two start scenarios. I base this on the current Power BI offering from Microsoft. Please see the official site for details, http://www.microsoft.com/en-us/powerBI/pricing.aspx.
* 40% off promotion for Office 365 E3 or E4 customers. Offered through December 31, 2014.

 

1) I have an Office 365 Enterprise E3 or E4 subscription, and want to add Power BI functionality.

• You can add Power BI functionality for the discounted* rate of $20USD per user and month, standard price is $33USD per user and month. On the Buy Now site you just choose "Add-on for E3/E4 Subscribers".

2) I do not have an Office365 E3 or E4 Enterprise subscription, or do not want to use the one I have.
You can choose between two options, "Standalone" or "Standalone + Office 365 ProPlus". It all comes down to if you have the "Office 365 ProPlus" or the "Office 2013 Suite" or not, this is what I call the client tools.

• With "Standalone" you get a SharePoint Online but you need your own Office-suite. Current offer is $40USD per user and month. 
• And if you need the client tools you chose the other, "Standalone + Office 365 ProPlus". Current offer under the discount is $52USD per user and month.

After this response I got a follow-up question.

?) Do I need a Power BI license for every user?
It all comes down to what the user needs to do. Once again there are two categories.

• "Publishers" are the users that create and share Power BI content, these users need a Power BI license.
• "Consumers" are the users that consume the shared reports, and they just need a license for the SharePoint Online portal.

Is this a full Picture? Nope, this is basically the view right now and it might change over time. If you are going to Power BI I strongly recommend a session with an Expert that can evaluate your current need and help you decide on what to get or not.