building .net applications without Visual Studio

Believe me without an IDE you just waste your time and it is a real pain !

if you feel that visual studio is expensive you can try the express editions or go for MonoDevelop or sharpdevelop

You don't need to be using another IDE to use a different build mechanism.

Not using an IDE is really a waste of productivity. You could use sharpdevelop or monodevelop though if you don't want to use Visual Studio.

I don't know anyone that is using NAnt or MSBuild instead of the IDE. Everyone I know that uses one or the other is using those tools in support of and in addition to the IDE. It's not that you can't write .NET code without and IDE such as VS, Mono or SharpDevelop, but they will save you a world of pain along the way.

Also, NAnt is no longer being developed or really supported. The last beta was something like two years ago. I would not recommend using it, though I know many that swear by it (and we even have to use it here for certain legacy projects).