Microsoft has a 3-year release cycle for their products. You'll end up paying every three years. For every single product. Microsoft's biggest cash cows are it's OS and Office products and always have been.
> A W2K12 Standard server license costs around 850 USD (~ 700 EUR)
Assuming you have just one server. At the least you will need a production server and a development server. And at least one for every developer. That's one OS license, one SQL Server license and one Visual Studio license per developer. Personally, I love work with a lot of virtual machines off Vagrant for development, testing and deployment. I am not sure how that works with MS licences.
> if it counts 5+ machines [...] some may consider to hire a network admin
It's fairly easy to manage ~100 machines using Ansible, Docker, Chef, etc. But in the Microsoft stack you probably want to keep the number of machines much lower.
> scale up or scale out
Scaling up is the right approach on Windows platforms but here is the issue I have with it. You cannot add RAM on the fly, as and when you need it. Nor can you add more processor cores on the fly. With horizontal scale, it's a no brainer but with vertical scale you cannot scale-up temporarily and then scale back down either. This is quite common and desirable for web applications.
I was on BizSpark and I was really grateful for the free trial. However, ultimately I shifted away and I am glad I did, even though C# is awesome and I absolutely love Visual Studio, I moved away for a number of compelling reasons.