I personally have a lot of problems with this.
First of all, none of the people involved has the money to buy the frag by itself. Obviously each of you agree that it is not worth $1000. Based on your math, in 6 months you have 4 frags which you can then sell for $4000. If that was indeed the case, you should not share the frag, you should get it for yourself. If you share the frag you can't get 4 frags until a year from now (after 6 months you get your frag and after 12 months you have your own 4 frags). So no one obviously wants to pay $1000 for the frag because of the inherent risk.
Now, if it is not worth $1000, why is everyone who pitches these things always reminding us that we can sell our frag for $1000? If that is the case, then no one is actually planning on growing out the coral, they are just in it to get their hands on it and charge the next guy $1000. So the whole thing only works if you're not the sucker holding the bag. That is the definition of a ponzi scheme.
The market for corals extremely small, and there is a huge amount of risk in doing this. Let's say another coral stings it or falls on it. You think you're going to be friends with the guy after that? If you are so confident in the sure fire return, you buy the coral yourself or at a minimum give everyone in the group insurance or their money back at any time. Anything else and you're trying to get something with a lower risk than what you bought.
There are a bunch of websites that specialize in these sorts of deals. The people who wanted them here know where to go to find them and know that there are indeed a bunch of people there willing to get involved in something like this.
Lastly, nothing is worth it until you pay for it. The second you buy a frag for $1000 you just reinforced the price and relieved the pressure on the seller to get rid of it. Honestly, those guys are laughing it up when they find someone to buy a $1000 frag, it just confirms that their gamble paid off and they'll try it again.
Corals are expensive, and I understand that there are indeed market forces causing prices to be in the hundreds of dollar range for a colony. But $1000 for a single frag? Who do you think is really making out on this deal, the buyer or the seller?