Depends a lot on the particular keyboard: I've seeb usb keyboards going from 20 to 500 mA. Useless to say the higher the rated power of the keyboard the bigger the impact. Also note that what's printed on cheap keyboards is not always true. I found a small usb keyboard at a local store here (some noname chinese brand) rated 25 mA that drained a full 500 and so was unusable with the tablet.
Although knowing how bluetooth on (and conected to the keyboard) affects battery would be nice... anybody?