I monitor the MLS listings for central Austin (MLS areas 1A, 1B, 2, 3, 4, 5, 6, 7 and 9), the area covered by the McMansion ordinance. I track 2500+ sq. ft. single-family homes listed for less than $300,000. (I explain why here.)
As of today (October 20, 2006), there is exactly one such house. It's on Saucedo, near Springdale and Airport. That's not really even central Austin.
There are no longer any "large" houses in central Austin affordable to families making $100,000 or less per year.
