I do know that there was a study conducted which found a significant increase in energy expenditure for 24 hours after only one dose. That's about all the info I have, however.
More importantly though, it's important to understand that even if there were a figure available, it isn't totally reflective of what's going on. Where exactly the calories being expended are coming from, inhibition of lipogenesis, and multiple effects upon gene expression, need to be considered.
Also, giving an estimated percentage in this case, probably isn't the wisest thing to do as it would be very easy to be "off" considering that you're basing the data off a relatively small amount of people. The response, metabolically, to such compounds is highly variable so to give a figure, without having averaged it over hundreds and preferably thousands of individuals, is going to be a mistake.