I have based my wedding off of previous trips. June of last year, I booked a trip and kind of left the group thing open to ANYONE who wanted to come. The grand total ended up being about $1200 pp to stay 5 days, 4 nights at an AI in Riviera Maya, Mexico... this was flying out of our small town into the closest big city (3 hr drive there), then flying to Cancun.
When FI and I went in 2007, it was about $900 pp flying from the bigger city. Sure, it saved money, but we had to pay gas, to leave the car parked a week, and drive 3 hours home after flying back! It was worth the couple extra hundred to fly from 5 minutes from my house.
ANYWAY, I'm rambling. We had 15 people sign up and come on the June trip! That was just for fun/vacation and that many people actually came along (many others were interested but things came up). I figured it if's for my wedding, even more people may find it worth their while!!! Everyone wished we had stayed longer for the last trip.