My opinion would be 15% if it's just the block that's missing. More if all the important peripherals are randomly missing along with the block. If it has any dealer paper to support the CE swap and all the important hardware and peripherals are intact, then maybe claw that back to 10%-12%.
Really needs to be assessed on an individual basis. Could be +/- a few points either way depending on the scope of what's all there and exactly what isn't?
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I like solid lifter cars, big cams and cars w/ 3 pedals in them. 
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