We spent 2 weeks in Italy in September, the second week we were in Tuscany for the wedding of my husband's cousin. I don't know all the costs, but here are my thoughts more generally. I think 26 people made the trip.
The bride and groom facilitated a villa rental in Poggibonsi that had a gorgeous outdoor space that they used for the reception, so no additional venue costs for the reception.
We paid for our own room ($450/person for the week) so their rental costs were minimal. The reception was catered by a local chef/restaurant, and was amazing. I have no idea what it cost, but that plus the transportation to/from the ceremony site was their major expense for wedding-day activities. They did plan and host another dinner and had a chef come in for that also. I think it was all arranged through the Villa owners.
The ceremony was held at the Abbey of San Galgano, outside Sienna. It was beautiful. I have never seen such a beautiful venue! It is open to the public during weddings (at least the when we were there) . quick google search says 1000 euro to reserve it. They had to do a TON of paperwork to make it official - both before they left and in the days before the wedding. It was a huge pain. The local mayor did the civil ceremony, and they had to have a translator.
In the end it may have been cheaper for them to do it in Italy than in their hometown of San Francisco for a large crowd, but I'm not sure. They brought/shipped a lot of stuff with them, and it was definitely a logistical nightmare at times for them (though they could have done with a lot less).
It was obviously a LOT more expensive and logistically difficult for us to coordinate than a long weekend trip to California from Ohio. We don't live in a hub area, so had to get to another airport in US, and can't get anywhere to Europe for less than $1000 typically (like other posters have commented). They did tell us in January, for a September wedding so we had plenty of time to plan. In the end it was a great family trip and I'm glad we went.
Also, I would appoint a friend or family member who is good at trip planning or leadership to be the key point person during the week. Trying to coordinate that many people without a "leader" was super hard, when only the bride had the info and she wasn't always available or thought she had shared but we never got word about events.
A good local wedding planner who is an excellent communicator and is on top of all the details is key. Their planner didn't tell them about the need for a translator until the day before, and she didn't make sure the bus with the guests went to the right place (so we missed Happy Hour in San Gimignano).