If its Company B's customers that can potentially win the trip, then Company B should pay more of the bill. That being said, who is receiving more value? If Company A is receiving more value by offering a trip to Company B's customers, then Company A should pay more. Another route you could take is with prize insurance. Create a competition where potentially no one wins - Ex. a hole in one during a company golf tournament. You pay a few hundred dollars and potentially someone can win a car. However, I think the answer to this question is based on who is receiving the most value. It doesn't matter if they are just email addresses.