I went through the same thought process just a little while ago with my new startup. I'm proficient in .Net but I kept an open mind and explored other web technologies. I thought I was somehow missing the boat by not using RoR or something open. But I had discovered what others have and that is that .Net scales just fine. I have government agency as a customer and they use .net and they have some very heavily utilized sites and the hardware spec to run them is quite manageable. Also there are legions of talented .net developers the world over. In terms of the development cost, the express versions of the tools are free and they are not missing features that you would need unless you have a big team. The server costs can be mitigated by joining the bizspark program. The other reason I chose to use the .Net platform is that there are 3rd party components that I require. I would rather use these natively rather than wrapping them in a web service or something like that. Personally I think the price between “free” and using .Net is in the noise floor.