Tony,
It all depends on the land value of the property. A lot in a bad area of a city can be worth less than nothing. Who is going to build a house on a lot in the ghetto when you couldn't possibly build it cheap enough to make money? Likewise, a delapidated house on such a lot is worth less than nothing, because the cost of demolition is more than the lot is worth and the cost to do a complete "to the studs" rehab would be cost prohibitive.
Finally, building codes have changed over time and some lots are too small to allow for bulding under the new building codes. So, if there is a condemned house on the property, once it's torn down, you aren't allowed to rebuild. However, you WILL be allowed and required to keep that lot mowed and cleaned up FOREVER! You couldn't pay me $1,000 to take a property like that!
In other areas, supply and demand haw lowered the price to the levels you've seen; such as $7,000 for a house. If the automakers are allowed to fail in Detroit, $7,000 may be the high end for many of these homes. In a city where the population has been leaving since 1950, who is going to buy a house and what would they do with it? I've seen bumper stickers that say "Will the Last One Out of Detroit Please Turn Out the Lights"!
Mike