Wow, what a great, complex inquiry! Your final paragraph summarizes the deeper questions here. I've been nibbling away at similar questions for my whole adult life, and now that I'm in my 60's, I have some answers. Some are so superficial, I'm pretty sure they are not the "final" answer, if there is any such thing. But here goes:
Chapter 1: In which the protagonist wonders why some people are negative.
All higher-order creatures come into the world with genetic variations that can cause radically different responses as a result of the same stimuli. I notice, in a box full of puppies, that one is intrigued by a strange sound and starts looking around for the source, while another may tremble with fear and back into a corner. Some creatures easily express joy; some seem to assume the worst. Even more so with people, plus we have all these complicated human concepts to wrestle with (some of which you are dealing with in your post).
Then throw in different actual experiences life hands us. For instance, I grew up in a desperately poor family, as were my 3 younger sisters. We have radically different takes on many aspects of our early lives together. Being poor didn't mean much to me until I reached high school and couldn't dress like the popular kids. But one of my sisters was incredibly status-conscious from an early age, and boy, did she suffer for it. I don't think that was her fault – her brain's just put together that way.
If 100 people hear the same neutral comment, like, "I couldn't afford that," there will be 100 different reactions, maybe "What a loser she must be," or "Well, it's not MY fault (feeling guilty)," or "Sure she could, if she worked hard like me," or a simple, factual, "I see that that is true for her," and so on.
There are lots of things I can't afford, and in most cases, the reasons are my business, so even if I comment on it, it's nobody's responsibility to do anything about it. My priorities don't include vacation travel, but I can happily choose less-expensive but extremely rewarding things, like gardening, or even my low-paid work as an illustrator for my husband's small non-profit publishing house. So if I were to tell you, with surprise, "Wow, that must be expensive!" it would be simple wonderment for me, not a reflection on you or your priorities.
So I'll bet it's mostly surprise as your friends "try on" the idea of taking a cruise themselves. There could also be envy, awe, jealousy, displeasure, delight, hatred, sadness, in various combinations. None of that is your responsibility, unless you intended to hook them on envy. It doesn't sound like you're doing that. But it is good to know which friends will share your delight and anticipation with you, and which will run in more negative channels. (Those would be the people with whom you share less, of course.)
Something I've learned, through many years of working to heal my own childhood dysfunctions (I grew up under the tutelage of an emotionally dysfunctional mother), is that other people do NOT "make me feel bad." It appears that they do, but really, it's my own thoughts about what's been said or done that makes me feel bad, good, or whatever I feel.
If another person kicks me in the shin, of course my shin will hurt, but after my initial yelp of pain, I might, if I'm in a good heart space, simply note that that person suffers real unhappiness because of his unconstrained behavior. Or notice he is feeling so bad himself that he's acting out, and I MIGHT just be able to care enough to forgive and respond with a little tenderness. That's actually fairly easy to do if he's a child, but most of us adults are just aging children.
That's my goal, to be able to live in a kind, generous, less-judging space more of the time. Sometimes I'm there. Often I'm far from there – but I'm getting to where I notice my negative thoughts more quickly, with practice. I've gotten tremendous help in this journey from a process called The Work, which anyone can investigate and try for free at www.TheWork.org.
So you ask what you can say to less-than-positive comments. Maybe nothing. Maybe just agree, "Yes, it is going to cost us quite a bit." True, yes? There's real wisdom in knowing you can't make other people wish you well, any more than they can make you not enjoy your opportunities. Although people sometimes try to do both.
Chapter 2: In which the protagonist suffers from guilt.
Guilt, when it's an expression of conscience, tenderness, and an acknowledgement of life's inequalities, is a good thing, and right now, it appears to me that this nation could use a bit more of it. It's not comfortable, but it is informative, and a deep, honest source of our humanity. I've known some seriously struggling people whose luck is all disastrous, no matter how hard they work, and they deserve my sympathy, and whatever help I might be able to offer. In fact, I often believe God put these people within my reach to help me understand gratitude, responsibility, and simple, Christian love.
I'm so glad you appreciate that some of your good fortune is a matter of luck. In spite of our small income, my husband and I live on a beautiful piece of land on a creek. We had the opportunity, skills and physical health when we were younger to design and build a tiny but charming and solid house with our own resources, no mortgage, on land that my mother made available to us at a price we could afford. (She wanted me living next door, too.)
Visitors often wish out loud that they could live in a lovely little cottage on a creek when they see our home. I don't take those comments as envy, or depressed sighs about what they don't have; I recognize that they are just telling their true feelings in that moment. I agree with them. But the location, size and quality of our home are some of life's "options," and is obviously not a "need" that those statements of longing are expressing, because those folks generally are living fine lives of their own choosing, effort, and good fortune.
There are other, genuine, needs that people are struggling without, that do warrant my attention and caring and work to correct. We all share those needs – affordable shelter in clean, safe neighborhoods where families are raising the next generation. Access to nutritious, fresh food. Basic health care that everyone can afford. Education that meets at least some minimum of quality. Access to transportation, and sufficient employment.
If I didn't have a conscience, or stayed too busy protecting all the "rewards" that I have personally "earned" in my life, I would ignore problems in the larger world that need some of my attention, effort and dollars, before they mark a whole generation of kids who will grow up believing in a dysfunctional, dog-eat-dog society with no vision or hope of improving things.
So I hope that at some point, all of us humans will pay attention to those nudges of guilt, right alongside feeling gratitude for what we do have. I will never believe that a rich, powerful man living in a gated mansion is simply not affected by the desperation of the poor, which are a growing class in our "classless" society. But daily, I do hear and see people of comfortable means beating their consciences down under the influence of fear – of what personal comforts they may have to moderate. And that, in my opinion, is one of the basest of human reactions. Our souls may fit through the eye of a needle, but our estates certainly will not.
So guilt is a healthy and necessary nudge, essential to a healthy and connected society. It means we have a functioning conscience. And while guilt can be rooted in fear, can be overdone, conscience is rooted in love, a more positive resource. It's just good to notice when guilt tips into the unnecessary or depressing feedback that keeps us from fully appreciating our good luck and happy opportunities.
I hope you enjoy every minute of your cruise!