Yes, to have a wedding on the grounds is $100. But, you can't have more than a few chairs if you have the ceremony any place other than the picnic shelter. I do not believe in making guests stand, so we rented the picnic shelter so that we could bring in chairs. For a full day, non-resident it's $300, half day is $150. Add $50 for a deposit but you get that back if you leave everything nice and clean. If you are a Cape resident, it's about half that price (we have a family member who is a resident who booked it for us). If you rent the picnic shelter, you do NOT have to pay the other $100 to have the ceremony there. So for $100 (they went up in January - we booked in December) we got the picnic shelter for the entire day. If you don't care about chairs and want to have the ceremony out in the open, there are a bunch of options: over by the mansion, amphitheater, down by the beach, on the grassy hill, etc.
One of FI's aunts hooked us up with chairs, but there are lots of places around that rent/deliver. The picnic shelter has a power outlet, so we're bringing our own music to play during the ceremony. The girls are arriving in a limo. We are able to have the ceremony at our reception site if the rain is bad, but we can also all fit under the picnic shelter and still have it there if we wish. That's a bonus of reserving the picnic shelter - it's completely covered and pretty big.
One downside of it all is that you can't reserve until January 1, which is usually when they go up in price. Their wedding permit was $40 last year, and went up to $100 on January 10. If you know a resident to do if for you, you can reserve on December 1.