I also used to also buy new headphones (with about the same budget) every 6 to 12 months, usually with the cable breaking somewhere near the connector. I had similar buying criteria as you, but just to test a hypothesis, I spent a weekend researching what to buy and arrived at the MDR7506 http://www.amazon.com/dp/B000AJIF4E for many times my usual budget. However, they've lasted for 2 years of being shoved in a bag or pocket many times a day and running to lectures, and still show no signs of wear, so they've almost paid for themselves in dollars per day even if they didn't sound better than a $20 pair which, of course, they also do. Highly recommended.

Edit: I see other comments also praising these. Yep, we're clearly a knowledgeable lot. :)