Q:

Scaling cheaply: MySQL and MS SQL

How cheap can MySQL be compared to MS SQL when you have tons of data (and joins/search)? Consider a site like  stackoverflow full of Q&As already and after getting dugg. 
My ASP.NET sites are currently on SQL Server Express so I don't have any idea how cost compares in the long run. Although after a quick research, I'm starting to envy the savings MySQL folks get.

A:

MSSQL Standard Edition (32 or 64 bit) will cost around $5K per CPU socket.  64 bit will allow you to use as much RAM as you need.  Enterprise Edition is not really necessary for most deployments, so don't worry about the $20K you would need for that license.
MySQL is only free if you forego a lot of the useful tools offered with the licenses, and it's probably (at least as of 2008) going to be a little more work to get it to scale like Sql Server.
In the long run I think you will spend much more on hardware and people than you will on just the licenses.  If you need to scale, then you will probably have the cash flow to handle $5K here and there.

A:

The performance benefits of MS SQL over MySQL are fairly negligible, especially if you mitigate them with server and client side optimzations like server caching (in RAM), client caching (cache and expires headers) and gzip compression.

