Whether a lens is worth X-time over another is highly subjective. The difference in price is really very difficult, practically impossible to be translated directly into performance difference. What you amy consider difficult to justify, to some it would be a no-brainer.
Another aspect you may want to consider when buying lenses would be the resale value, ironic as it may seem. If you buy a popular and fairly sought after lens, you can sell that later down the road if you want to upgrade with minimal loss, if any. Case in point, the 70-200mm f/4.0L IS is ever so slightly better than its non-IS version (some folks may not agree with that statement and for right now, consider them to be equal) but it has the IS feature that will let you take images with less worry about blurred images due to camera/lens shake. That in itself is a great feature that would lead to better images, although not directly due to a better optical performance. There is a price difference of approximately $400 between the two. But it is easier to sell the IS version of that lens and furthermore, it tends to retain its value better than the non-IS version. Good luck!
Hope this helps somewhat,
Joshua
Jan 18, 2012 at 12:33 PM
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