A while ago there was a thread about affordable eco housing, here is an interesting possibility for the price of a normal well built house.
http://www.guardian.co.uk/environment/2008/feb/28/zerocarbonhome
http://www.zedfactory.com/zedfactory_how.htm
From 1500 euros per square metre, the cheapest low cost junk houses start at about 800 per metre and a well built custom house is about 3000 per metre.