The number is going to be very different from one person to the next, probably several times higher or lower in some cases. You can't just look at the biochemical factors either (like metabolism, body temp, activity level) you have to look at the neurological factors also, like fidgeting, squirming around while sitting, etc. Some scientist is studying these "micro-movements" that are related to metabolism. Are they a cause or an effect is the question, but either way they still burn calories.
If anyone knows that study on micro-movements, post it, I can't find it.