Brett, Greg gets to keep at least 40%, you are just paying for the experience, which would be well worth the money. If you want to invest more, you can keep some of what you find, and it's still cheaper than buying most certified coins online.
Our salvage group got started in much the same way on a different boat, we were very lucky and were invited for free as friends. Still, getting to do this is an experience you won't soon forget, and you won't find many "tour guides" as good as Greg on the 1715 fleet. It's a great idea, and a wonderful way for those who can't drop everything and become a treasure hunter, to experience the thrill.
Jason