Higher end (>4k) bikes seem to go for low prices compared with their purchase cost. Looked at as a percentage of original value, they go for a deep discount. I think part of the problem is finding willing buyers for high end bikes used. I would guess that most people looking to spend 3-4 grand or more on a bike are going to visit the LBS before buying used.

However, even crap bikes not worth $10 seem to fetch $50-100 on craigslist around here (washington DC).

If you made a graph of the used bike market around here then it would probably look like a rapid decline that levels off around a certain value and stays there indefinitely. For example, an aluminum frame cannondale in working condition with STI shifting would probably fetch 300-400 dollars at a minimum. Whether this was a mid 1996 or a 2006 model wouldn't matter--those characteristics guarantee those prices.