Prices have come down but it really depends on the location of the condo within Logan Square and the age of the building. Over the last 12 months for example, 7 2br/1.1ba units have sold ranging in price from $50k to $350k.
Logan has several areas and some have held values better than others. Living and working in logan for the past 8 years i have vast experience assisting both buyers and sellers. happy to chat offline if you'd like.
The Case Shiller index may not be perfect but it employs a much better methodology than simply taking averages. The shortcomings of averages are well known and so telling someone to ignore them is not an ignorant statement. They are too heavily influenced by mix variations.
"Pay no attention to averages" is an ignorant statement. One can find a weakness or hole in any argument. The Case Schiller Index uses information of home sales that may be skewed due to homes being sold before and after an HOA goes insolvent for example which would greatly influence the index numbers. "Flipped" homes which may have sold for less say in 2007 and have sold for significantly more in 2012 will greatly skew an area's numbers....
Pay no attention to average prices. They are heavily influenced by the types of properties sold. For instance, today's average might reflect much higher quality units than the averages from 2007. The Case Shiller index which I reference on my stats page does not suffer from this problem. It reflects same property sales.
It can be more than 50% or much less. You cannot generalize too much in most neighborhoods including Logan. You need to have a few agents do a market analysis based on your property to get a fair estimate of current value.
I maintain a page of Chicago housing statistics here: http://blog.lucidrealty.com/chicago_real_estate_statistics/ Since September of 2007, when condo prices peaked in Chicago, the average condo has declined in price by 39.3%. Now that's on average. It all depends on the location. The farther east you are in Logan Square the better off you are from a price decline standpoint.