Regarding ASP .NET, well the framework is free but Visual Studio is what will cost you some money. Depending on what position you're in you might have some options. If you're a startup or a new company you can apply for BizSpark. That'll get you free tools and Azure for 2 or 3 years. Microsoft will essential support you as you build out your product with their tools, but after that you'll have to start paying for them. If you don't mind being on the bleeding edge, there is an open source version of ASP.NET that is pretty fast and runs cross-platform (linux, OSX, windows). Bundle that with Visual Studio Code, their cross platform editor, not IDE, then you can start building your solution.
Regarding performance of the platform, you can check out the benchmarks repo of their Github and you'll seem some fairly impressive perf stats in comparison to some other frameworks. At the time I'm writing this though, the framework is still beta/rc quality so it's a work in progress. However, for an example of some folks doing real work with the framework, check out age of accent. They use the open source version of ASP .NET to power their multiplayer game. Check out their Dev Blog too to get some more interesting stats.
I'm not a SQL Server expert, so I won't comment on that. However, you can use any database you want with .NET and not just SQL Server.