edit to clarify: I am wondering if the amount that is actually consumed is negligible in terms of nutritional information.
From a pure estimation stand point it will vary too much batch to batch. If you let a batch sit a week longer than your last batch more yeast will have flocculated out. If you use a highly flocculating strain you'll have less than a poor flocculated. Temperature and vibrations from the environment will effect the sedimentation rate.
The only way to know is to count it with a scope and hemacytometer.
The amount of yeast in grams/l will most likely be negligible. If you want to now exactly how much yeast is in your beer, you have to do cell counting, which is a bit involved and requires a microscope plus additional equipment. White Labs has a description of how to do it.
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