Suppose you get all 5 million questionnaires. It would cost you $5,000,000 x $0.42 = $2.1 million to post them all separately. In return you'd win $3,000,000 dollars. Sounds a good deal.
Now think of it in terms of expected return per questionnaire. The chance of yours being picked is 1 in 5 million. The expected return based on a $3,000,000 prize (assuming each questionnaire has an equal chance of winning and all take part) is $0.60 dollars ( 3 million return x one in 5 million chance). The expected return of $0.60 is greater than the cost $0.42 of the postage. So it is worth it.