This article was all over the map in terms of numbers. Here at Expat, we develop on the Microsoft stack whenever we have a choice in the matter, including for our own products such as Twiddla, Blogabond, Rootdown, S3stat, etc. and here's the breakdown of what we pay for what:

Tools: $399 total. For Everything MS makes, for every developer in the shop, once every 2 years. That's what it costs for an MSDN license through the EmpowerISV program.

Servers: ~$2000 per production box. That's Small Business Server, which comes with production licenses for SQL Server, Exchange Server, and pretty much all the backend stuff you could want.

And that's it.

Assuming we buy a new server every 2 years, that means we pay about $100/month for the privilege of developing on the MS stack. Put another way, that means that the MS stack need only gain us 20 minutes worth of productivity every month to pay for itself.

Like I said up top, we build on the MS stack whenever we have a choice in the matter. It's just a no-brainer.