<p>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&amp;As already and after getting dugg. </p>

<p>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.</p>

I know that stackoverflow has had problems with deadlocks from reads/writes coming at odd intervals but they're claiming their architecture (MSSQL) is holding up fine.  This was before the public beta of course and according to Jeff's twitter earlier today:
the range of top 32 newest/modified
  questions was about 20 minutes in the
  private beta; now it's about 2
  minutes.
That the site hasn't crashed yet is a testament to the database (as well as good coding and testing).
But why not post some specific numbers about your site?