I don't know what could be the reason you consider only 0.5 bitcoin of the tickets sold as "real" and the others are "given away" but in reality I honestly believe none of it was given away, these are money you may not have seen if you didn't had the contest so its all a profit for freebitcoin if you ask me.
All you need to do is look back at the site's history of past lottery draws (yes that's right, it's all available for everyone to see and is one of the best damn things about this site). Before 1 May, the date of the Lambo draw, people were still wagering and buying lotto tickets. So it's clear for all to see, there was money to be seen even before the contest started. As TheQuin says, hard to ask for hard numbers because it's subjective what a promotion does.