What's so expensive about MS development? SQL Server Express still allows 10GB and is free. There are express versions of Visual Studio if cost cutting is the end-aim.
How many companies/developers only use the low-end "free-ish" tools from Microsoft? Have you looked at the limitations? Have you looked at the hidden gotchas in a program like BizSpark? Taxes, license costs, API churn and version churn can bury smaller companies or dent a budget in a division of a larger one.
When working with local smaller companies that are not solely mobile software sales or consultants, almost all the newer ones have a large percentage of OSS in their development and hosting mix. It could be R or Python for statistical analysis. Linux for hosting. MySQL or Postgres for RDBMS. Git for source control. Kettle for ETL. Multiple different places to host.
And larger corporations are adopting, investigating and actively supporting many of these tools. Plus you can get support agreements from companies like Redhat or Continuum Analytics if you need assistance or additional software.
As a developer/admin/database guy that works and lives in both the Microsoft and OSS worlds, there's a lot to admire about both. But increasingly there's only one that I can afford to support.