Microsoft's stack adds a not-insignificant cost to a 3-4 person bootstrapped startup's budget.

Up to you whether or not you're getting enough of a productivity gain to justify it.

Visual studio express editions are free and a windows vps is cheap.

Even if you decide to spring for the full developer licenses you are looking at less than a week of a developers salary for some really good tools.

During one of the podcasts I believe Jeff stated that he was able to get free copies of the most expensive version of Visual Studio from a friend at MS.

How much would SQL Server licenses cost?

I'm not sure if you're being sarcastic or not but I'll give you the benefit of the doubt:

http://www.microsoft.com/sql/editions/express/default.mspx

If you are a startup you can probably get Microsoft's software for free.

Talk to one of their technology evangelists, they love seeing cutting edge companies building their business on MS.

Anand Iyer, http://blogs.msdn.com/aniyer/, can point you in the right direction.

Jeff Attwood and co that built it are used to .Net/MS/Visual Studio development, so for them the cost was probably small compared to the cost of switching stacks + tools.

The MS stack also has lots of good tools which can help speed up development.