Basically, deploying a .Net site requires at minimum 1 personal license of Windows and 1 server license of Windows. Sometimes a non-free copy of Visual Studio, and usually a license for SQL Server (since many MS people use the whole stack for integration reasons). Without buying hardware, Windows hosting is also always more expensive.
Compared to a LAMP/Rails/Django style app which you could theoretically get off the ground for the hardware cost of a local PC and cheap VPS hosting.
Of course developer familiarity, your local hiring pool, initial launch scale, business partners, vendor integration, etc, etc, etc, all influence the tech choice and its impact on cost.