Cultural differences here, Witz. What is convention in your area is not the same convention in other parts of the world. Also, times are changing and some people are keeping up while others still have old-fashioned expectations.
What happened in our family - we told our kids, "We can give you $10,000 towards the wedding. After that, you pay for it."
With difficult child 1, his wife's family also kicked in $10,000. The wedding was done on the cheap at our local church (on the beach in the village). The caterer donated her services, we just had to pay for the ingredients. The booze was bought on appro, what wasn't used the store bought back. Serving staff were also friends and volunteers. The wedding "car" was the bride's uncle's boat, they moored at the village wharf and she walked across the beach barefoot in her beautiful bridal dress. Total costs - about $2000 max including the dress (which was made by a friend of mine). So our rule - the kids got to keep any leftover cash.
easy child was next. She and SIL1 organised everything, paid for everything and accepted the $10,000 from us to put a deposit on a house. SIL1's mother was unable to contribute anything, she's on welfare in public housing. But the kids knew that, which is why they worked hard to pay for it themselves.
easy child 2/difficult child 2 - her in-laws were unable to contribute much if anything, so she budgeted to the limit of the $10,000 and saved and scrimped for the extra. She cut costs where she could, called in favours, did deals, shopped around. I was with her for a lot of this. She made all the bridesmaids' dresses (with my friend helping again). My seamstress friend was also MC (for both girls' weddings) and we made it work.
Three weddings - $30,000. If husband had not made the offer that he did, we would have saved about $8,000 on difficult child 1's wedding and the full $10,000 on easy child's. Kids these days, especially when they are managing on their own independently, tend to pay their own way. At least in our neck of the woods. But for example with easy child 2/difficult child 2's wedding, her in-laws were taking the old-fashioned approach which is that parents of the bride pay for the reception, the dresses etc and the groom (or groom's family) pays for the cars, the flowers and the booze. And the photographer. As it happened, we provided our own cars (saving money) and a friend of SIL2's mother was the photographer at a discount rate.
You need to talk to the kids, find out what they expect. Before you talk to them, work out what you can afford.
Now to other weddings in our family - my sister has just had one wedding (her son) and in a few weeks, her daughter is getting married. They live in a beautiful part of the country, in the sub-tropics. My nephew's wedding was on the cheap, where possible he and his wife paid for what they could but both parents' families also covered what they could too. My nephew and his bride have been living together for 13 years, they live on a cottage on my sister's property. None of them are wealthy. My sister and her husband cover their costs by working as cleaners. But it was a beautiful wedding. Because of my experience with my own kids' weddings, I saw where corners were cut. They hired professional caterers, my sister's twin made a mound of cupcakes which became the wedding cake. The wedding cars were provided by friends, the bridal party were the couple's four kids. Best man was their oldest son, 12 years old.
We had to travel to that wedding. We decided to have a holiday in that area, we used time share (saves money). Other family members also stayed in the area at least for a few days. Family who lived near enough (one brother and another sister) commuted from home. My sister (mother of the groom) organised a local villa where three families stayed for a couple of days.
Accommodation - B&B tends to be cheaper, check it out. Resorts can be expensive and cheaper hotels/motels are often really not that comfortable or nice. Like flop houses. You want somewhere nice, but not necessarily ridiculously luxurious. B&B is a good compromise.
Now, my poor sister - her daughter wants to get married on Magnetic Island. I've never been there, so I'm looking forward to it. But the last wedding was in March plus difficult child 3 has exams coming up, so we can't stay for a holiday. We're flying up for three days only, staying at a B&B. I'm sharing the B&B with the brother of the bride (another brother, not the newlywed from March) and another sister of mine and her husband. Our younger brother (who has plenty of money and doesn't understand those who don't) is paying twice the price for an apartment down the road. WE get breakfast thrown in and our place looks just as lovely (from the photos).
The wedding reception will be in a top-range hotel on the beach. My sister (mother of the bride this time) said it's about $180 a head. The kids are paying at least some of it, I believe, plus the guest list has been whittled down. difficult child 3 is coming with us but is not a guest. He will stay alone in the B&B probably (supplied with pizza), unless my sister wants him along to keep his younger cousins entertained as he did at the last wedding. The cost for us to go to the wedding - about $1000 for three days including accommodation and air fares.
If the wedding is held at a mid point between NZ and the US, everyone has to travel. The advantage is, costs are equal, everyone is equally inconvenienced. They are also already on the spot for their honeymoon and you can enjoy a holiday too. The worst cost is often getting there.
The disadvantages - everyone is inconvenienced. Your guest list is curtailed. If you've forgotten anything, it's too late now.
My vote - suggest they hold the wedding either in two separate places, or in one only. For example, a NZ wedding would mean you and your husband would have to travel but you're up for travel costs anyway. You might be able to be billeted with groom's family in NZ which could cut your accommodation costs, and you could also have a bit of holiday while you're there. NZ is a beautiful place to visit. Or you could host the wedding, billet the groom's family, show them around. The kids could then fly off to their tropical island (entirely their expense, the honeymoon) and everyone is happy.
Seriously - if you have time share, or you have a friend with time share, see if you can access a gift certificate for a time share place near where the wedding will be. Failing that, look for B&Bs and check out prices. We've found time share costs for a week, what a similar place would cost for one night. And depending on what sort of place you book, you can share it with others to cut the cost further. For my nephew's wedding in March (when we thought the other kids were coming too) we booked a three bedroom place. With only three of us as it turned out, we rattled around in it. But it still only cost us about $350 per week because we own time share. We bought bonus weeks. We did the same four years ago when we went to NZ (which is why I'm really recommending you consider flying the extra air miles, you would love the place).
Sorry this is so long, but I hope there is stuff here that can help you work out what is best in your situation.
Marg