I'm also looking for something similar, but different because I don't want it to be instant. I'm looking for the average weekly BTC price between all trustworthy exchanges. Daily or monthly values could also be useful.
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If I end up putting one together I want that sort of functionality on it as well, with all the historical records. Also thinking the deviation of each of the exchanges during that time period would be a neat feature (i.e., Gox is 5% above average, Bitstamp is 3% below, etc.) And the changing percentage of each exchange over time. Just need some time to work on it...