May be 30% is the right number, but given the lack of actual link to the study and the basis on which they define the 'true value', it's a tough call.
Just for a little perspective, visit this site of an Edmonton realtor. For some reason she has properties from 3 or 4 years ago still listed at the then selling prices. Look at some of the prices. 1500 sq feet Semi-detached homes selling for less than $150k. Even going as far back as 2005 and early 2006.
It's just amazing how people tend to lose perspective on where things were just a year ago. Or two years ago, never mind going any further back. Here's a refresher on the Edmonton home prices again:
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Will prices fall by 30%? Perhaps. Will they not fall at all and only remain the same for the next 10 years and let inflation do the trick? It's a possibility. Will they go back to the levels seen in 2005 or 2004? Why not?
Does the above graph provide any answer on how hard the fall can be?