The thing is, if you already decided on MS SQL Server, why buy Tableau? The company I work for has both, and Tableau is the most likely to be dropped of the two.

Hello? The MS SQL Server license has been changed to be per CPU core. Consultants want you to create a seperate MS SQL Cluster (serveral bare metal servers with many CPU cores). So MS BI will be a lot of more expensive as you first think. Most see just the Tableau is more expensive than BI and decide for MS BI just to find out MS SQL is rather expensive. Also you need at least MS SQL Server 2014, despite dark pattern speak (lies) about that 2012 may work too. Oh and don't forget they will mention that you will need a seperate SharePoint cluster (web and app servers) to run Excel webservice and the SharePoint based dashboards. Oh and you will need to upgrade all your clients with Office 2013. And they will invite your CTO to an Windows 10 event and give away some WinPhone10 and tablets. And then you will have to figure out how to comply to local state law by upgrading to the expensive Windows Enterprise LTSB license.

So what is cheaper?

At least in the case I'm describing, the company already has an expensive MS SQL Server 2014 license (Core licensing) which allows for an unlimited (ok, something like 50) amount of VMs, and they already have the expensive bare metal servers, so that's not a concern, since it's a sunk cost.

All clients are already on Office 2013, so that's not a concern either.

The Sharepoint cluster probably is a concern, but that's about the only thing, I'm pretty sure it'll be cheaper than Tableau.

I wish they'd invite the CTO to a Windows 10 events and give us some giveaways :) . No local state laws, this is for an insurance company in South America btw :)

Edit: the opinions are strictly my own, I'm not involved in any decision making sadly (no Windows 10 giveaways for me :P ), etc.