Each batch of pictures you add to your cart just costs a bit less than the previous one:

Here is an example of how it works: 20 photos*$9 + 30 photos*$7 + 50 photos*$5 = $640 for 100 photos.
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Of course, you don't have to calculate that manually. The system will automatically calculate the total cost for you if you once you've put the needed number of photos in your cart.