Here are my back-of-the-envelope calculations. I account only for the energy expended in pulling your body mass up.
Say you are 70kg (approx 155lbs) and a pull up raises your mass up by 60cm (approx 2ft).
Then one pull up increases your gravitational potential energy by m × g × h = 70 × 9.81 × 0.6 = 412.02 Joules, which is very nearly 0.1 kcal.
Of course, the human body is inefficient. If its efficiency is 10%, then one pull-up would burn about 1 kcal. If its efficiency is 20%, then one pull-up burns 0.5 kcal. 10% and 20% are some of the numbers I have seen on some webpages.
Does anyone have a more precise account of this?