I am extremely wary of projected price savings based on the price of an individual item.  If the item is stand-a-lone then the price you pay probably is what was advertised on the box but an OS is an emphatically bigger commitment.  It is also a slippery slope.

Let's suppose you switch your entire server infrastructure to Linux.  From a developers perspective it now makes sense to switch your workstations to Linux or OSX.  However, not all your tooling works on Macs and Linux OS's.  All of a sudden you have to start scouting for alternatives.  If you find the alternatives do they play well together?  How much time do you burn because of the friction points introduced by new and different tooling.
What is the skills market like for the chosen tools?  Is it easy to recruit/educate and at what price? 
Are you looking to embrace open-source because it is perceived as a lower cost cheap option?  If you decide to go with commercial support those open-source tools start to look much less of a bargain.  What is the quality of support available anyway?
It's all about TCO - Total Cost of Ownership. . Perhaps on a balance sheet something seems to make sense but if you don't look closely at your friction points you can bleed away an immense amount of time and productivity.
If you are already in a mixed Linux/Windows environment I would be looking to experiment with SQL2017 on Linux.   If I was in a pure Linux/Mac OSX  environment I would be asking what tooling would be available to me and what advantage I would gain adopting SQL Server.  
It has to be said that the SQL Server community are exceptional.