I've been involved in several weddings in the past 10 years. In the past, it has always been customary in the area for the bride's family to pay for the wedding & the groom's family to pay for the rehearsal dinner. (Our family is a little different from most. When we have a niece or nephew get married, we all contribute to the cost & pitch in to do the work, which is why I use the term family.) The three weddings where that custom was followed went very smoothly. Those were a breeze. Another wedding, the bride's parents didn't want to pay for anything. My sister wanted her DS, an only child, to have a wedding & rehearsal dinner, so my sisters & I did all the work & paid for everything. The bride's mom showed up with a single fruit tray from the grocery store to be served at the reception.
The random tray of fruit was her only contribution.
Naturally, we thanked her for it & found a way to include it. For the others, the parents of the bride or groom that our niece or nephew was marrying wanted to pick & choose what they paid for. Those didn't go so well. It created a lot of chaos, when trying to make sure everything was done. My youngest niece's wedding was a disaster thanks to the groom's family choosing what they wanted to provide & insisting they be the one to decide what those things were. My niece wasn't consulted in this. She got what they wanted her to have. Her future MIL even insisted on providing the bouquet & didn't let her see it until rehearsal. It was horrible & made my very easy going niece cry. Honestly, any bride would have been upset over it. Fortunately, the groom's aunt was embarrassed that niece's future MIL would bring something that looked like that & took it to get it fixed for the wedding. We provided all but the three things they choose to contribute, but let the bride & groom choose what they wanted. We really would have preferred for them to have stayed with what is customary & provided the rehearsal dinner only, which they refused to do. So, in our experience, it's best to stick to what has always been customary in that area. Customs do vary a lot by area. If the bride & groom aren't both from the same area, there definitely needs to be a conversation between both families, before any decisions where money is involved are made.
My DS is the only one of the nieces & nephews who isn't married at this point. He's also the only one that won't marry someone from the area the rest of our family lives in. Our family is very much a DIY family for weddings, but DH & I had planned to have a rehearsal dinner at the restaurant of the couple's choosing, when he gets married. I was really looking forward to finally not have any work to do at a wedding.
Thanks to this thread I now know it is customary in some areas for the groom's family to provide the flowers. We'll be prepared to pay for those too, if asked. I'll most likely make the arrangements myself, which won't be a problem. I'm the flower arranger & bow maker in the family. We were also planning to pay for a reasonably priced honeymoon or give money toward a more expensive one. If it is customary in his bride's family to divide the cost between parents, we'll do that. If that's the case, I hope her parents are okay with us doing as much as possible DIY. I have a lot of family who are happy to help & I'd rather not pay someone else to not do as good a job as we would. We've gotten really good at weddings, after having so many family members get married in the past 10 years.
That said, if we help with more than the rehearsal dinner & flowers, the happy couple will be on their own for their honeymoon. Obviously, this is all speculation on my part. My DS isn't even close to getting married. When he does, we'll work with the bride's parents & do what is expected of us.