Some startups that used the .NET stack:

-Stackoverflow: http://stackoverflow.com/

-Agile Zen: http://agilezen.com/

The framework you use to develop doesn't matter. If you like .NET go with it, if you like RoR go with it. For costs, you can take a look at the BizSpark program from MS, you can get all MS products you need for $100 for 3 years, then you pay for it.