Yeah, ok, so this is really a mis-alignment in values. He is willing to sweat the small stuff so he can do the big stuff that he really cares about. You would like to be efficient in everything and so don't like spending on the big stuff that is inefficient (e.g., if the purpose is to go to this expensive place, let's do it when the kids are old enough to remember it; if the purpose is to be with family, let's do it somewhere cheaper). He is extremely conflict-averse with his family, you prefer to be up front and see no harm in asking for something that makes more sense. So you are arguing efficiency and logic, when he is focusing on emotion and relationships.
I think the key to compromise here is to find a way to speak his language. Recognize that he is not hearing "perhaps we can convince your family to look at a less-expensive place?" -- he is hearing "she thinks I am a failure because I don't want to risk upsetting my family." You think you are saying "let's find a way to pay for this," but he is hearing "I don't care about the things that matter to you."
The way to break the impasse is for you to figure out how to talk in his language -- talk about the emotions, not the math. The first thing you should do is get a bottle of wine. :-) The second thing you should do is spend the first, say, 10 minutes being very supportive of the idea of the trip and acknowledging its importance to him -- I can see how much you love your family, I love them too, boy I wish we were closer and could spend more time with them, I really want our kids growing up knowing their cousins/aunts/uncles. Validate his emotions. When we feel attacked, we get defensive and put up walls. This opening is really important to help him relax -- he needs to know you get it before he will feel like he can trust you to have his back on the things that matter to him.
THEN you can turn to your concerns. But again, frame them up in terms of emotions -- e.g., I know that you feel like we make enough. But I feel scared/insecure when we cut back our savings. You know I'm a planner; having a budget, and knowing what we are going to spend on what, makes me feel safe and secure. So when something like this comes out of the blue, I get scared and insecure. I need to have some order and control just as much as you need to be connected with your family. And I know I'm good at finding deals -- yeah, that's my superpower -- but trust me when I say I have looked here, and there just aren't any. So I would really appreciate it if you could help me figure out some way to keep this within our existing budget -- the most logical approach seems to be to cut the other vacations, but I am open to other things as well, since I know how important our time away is to you.
Or, you know, whatever your emotions actually are, vs. whatever it is I am pulling off the top of my head. :-) The key is that you start with showing him you have his back, and then you ask him to have yours.
Also: adjust your expectations. Yes, it is stupid and illogical and self-defeating not to even question family decisions or make other suggestions. But that's also who he is, and attempting to change that (at least right now) is only going to make him feel attacked and get defensive and shut down. So look at it as a personality quirk that is worth the rest of the package, and figure out another workaround.