paybitcoin ... not sure where you got your info
1) Startups can get free licenses using bizspark program for 3 years plus heavy discounts on conversion at "graduation". Any company older than 3 years is hardly a startup.
2) Your pricing but it is way off. Standard can be licensed per user (CAL) instead of per core which is much lower cost for applications like web backend datastore.3) SQL Server 2012 Standard is limited to "only" 64 GB of RAM but it is hardly a deal killer for startups. To my knowledge no Bitcoin venture uses a larger database than that (startup or note). Very few companies in the world do and they tend to be very large established enterprises spending seven figures on IT costs.
4) If the 64GB limit is a concern, SQL Server 2008 R2 is an option and it has no memory limit.
5) SQL Server standard has always supported mirroring (Safety Full). There is no additional license for a failover (passive) cluster. Additional licenses are only required for a load balancing (active) cluster.
6) While Windows 2008 is limited to 32GB of RAM. Windows 2012 standard has no such limit (well technically it is an OS limit of 4TB but that is regardless of license).
It's great as a technical solution, but has a bad value proposition as a web startup solution.
Which is why bizspark was created. Three years of essentially unlimited licenses. Then a graduation with conversion of licenses at 70% to 90% off retail. Any company older than three years is hardly a startup.Can it be done on other platforms for less? Sure but there is no reason for FUD.
Still asp.net can be use MySql (or any RDBMS) as the datastore. As you pointed out the windows license is downright cheap compared to SQL Server so that is a lower cost option. Microsoft's big competitor for SQL Server is Oracle and there pricing makes SQL Server look downright cheap.