SQL Server starts free and it was actually only halfway through last year that I stopped using the free version at Playtomic. It's also the only significantly expensive part - and very easy to not use in favor of something that is free/open source/cheaper/whatever.
I'm glad I went with .NET for my startup - a few days ago I calculated some current numbers: 29 million unique people logged over 3 billion events during a 10 day period. It's holding up well enough for me and my very lean, mostly Microsoft-flavored startup.
Yesterday I came across this writeup on what Quora is built on too: