Here's a less scientific but perhaps more meaningful measure. If you're looking for a house, what's out there for sale?
My search results (run on 1/29/07) are below. Here are the parameters I used: 1400+ square-foot; single-family; active MLS listing; $300,000 or less.
Rationale: 1400 sq. ft. is a completely arbitrary dividing line, I admit. I use it because it's the low end of what I think of as the "medium-size" house. Certainly, singles, couples, and empty nesters may all be comfortable with a smaller house. Households with, say, 4 or more members tend to prefer more space than this, though, so I use it as a rough measure of affordability for families. (Before I get flamed by all of you who are raising two or more kids in less than 1400 square feet -- I'm talking about typical preferences, not what is "necessary" or "appropriate" or "optimal.")
I know that $300,000 is not "affordable" under any conventional definition of that word. On the contrary, I use that figure because it is roughly the upper bound for homes within the reach of even "upper-middle income" households -- those making $100,000 or so, which is approximately 80th percentile in the Austin metropolitan area. This gives us a good sense of how little there is for the truly moderate income.
Here are the results:
South Austin (MLS areas 6 & 7): There are three 1400+ square foot, single-family homes listed for sale for under $300,000. None is larger than 1600 square feet.
North Austin (MLS areas 1B, 2 & 4): Better. There are 23 such houses (four between $179,000 and $200,000), most in the Crestview, Wooten and Highland neighborhoods. One of these is a veritable McMansion at 2300 square feet. Everything else is under 2250 square feet.
East Austin (MLS areas 3 and 5): Best. There are 53 such houses. (This total includes a number of homes from the subdivisions bordering 183, though, which some people may not consider "central Austin.")
I intend to update these searches periodically to track supply. Needless to say, I expect these numbers to get smaller.