If you have a great product that you can get live quickly and works then it won't matter if you even write it in COBOL (don't write it in COBOL though) - ASP.Net & SQl Server (even the free version) also work very well together and have great performance.

There are of course licensing costs if you run on Windows and don't use MONO but these shouldn't really be an issue in comparison to all your other costs

Also there are a lot of ASP.Net/C# developers around (just look at odesk) which will help you build your team faster (unlike Ruby or Scala developers, which are great languages, just harder to recruit and also attract higher wages).

My advice... build a proof on concept as quickly as possible - if .net is your thing then use that vs. wasting time learning something else. The chances are that your concept may need to be reworked so why waste time...

Good luck!