Just some data to throw out from my experience:

Level 70 pally
Level 30 lowbie
about 100 xp per kill

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Level 70 pally
Level 30 lowbie
Level 29 lowbie
about 110 xp per kill

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Level 70 pally
Level 55 shaman
Level 30 lowbie
about 50 xp per kill


I don't know the exact formula but as you can see, it was very beneficial having more than one lowbie of the same level in the group than two toons who weren't getting any xp plus a lowbie.