We have a similar situation, but with 2 'perks' so to speak. We are being offered $25 for each adult that books, and then also for every 20 people that book a free trip. What we decided to do was have the TA take the $25 off of our guests' price, and then we will take advantage of the free money towards our trip for 20 people booking. This way we figure we are passing along some savings to our guests, but will have some incentive to our trip as well. 20 people is a lot of people to book to get our incentive, so it seemed fair for our guests. At least we thought so. We are not advertising to anyone that we get any kind of incentive though, so no one should ever find out.