I don't think you can do it on a day trip. If I remember right, the day starts pretty early in the morning - about 8:30 or 9:00. I'd suggest getting over to Ian's early in the evening in time for dinner, have dinner with the other guests and listen to their adventure stories from the day, get a good night's sleep, and do the trip the next day. Then, after the rapelling, go back to Ian's, get a drink, have dinner, get a massage if available, and head back to AC the next morning. You'll be glad you did it, it's not to be missed. Search the archives under "other parts of Belize" and you'll find more info if you want to read more about other people's (including my) experience. Enjoy!
Oh, and it's $105 per person.