Apparently it was around $30 or $40. Of course, to my knowledge, vB itself has never gone on sale. I also don't put my software on sale because I think it rips off people who were a day or two off.
Some called it a tactic of desparation which I don't think was fair; besides, Chen sold quite a few licenses that day after reading all the posts here and at vB.com, so it seemed to work all right