I'll take a somewhat different tack on the situation. Yes, boundaries are important, and you should decide where those boundaries are, and enforce them. That said...
Your boss makes those comments because your behavior is outside the norm. And I see it as an opportunity to evangelize without getting preachy. For example, you could say something like "You're right, it's a lot. Every paycheck, my retirement moves two weeks earlier!"
That's an interesting approach. What makes me hesitant about that is it could cause envy and resentment in others.
It could, or it could not. It's really not effective to try and control people's responses to you though, you don't really have that power. The best thing you can do is just be clear in what you are willing to share and what you want to project about yourself.
You can't live in fear of people judging you based on what you do and say because there's no way to get through life without triggering negative feelings and responses in others.
Some people will judge you for what you have, some people will judge you for what you don't have. If you are genuinely a decent person, then only assholes will judge you and it's a waste of energy to worry about what they think.
Be a good, respectful person, have boundaries, and don't be afraid of being judged for living the way you believe in.
Listen to Malcat!
I have an example from my own life that might help. My friend recently made a purchase that I would not have chosen to make, even if I had twice as much money. I didn't say that, I just congratulated him on the purchase because I know he really wanted it and can afford it and generally makes good decisions and actually likes his job and doesn't to my knowledge hang around on personal finance forums where they frown on such things. Even if I thought he was making a really bad decision I would be very careful about the way in which I said something because it's not my fucking place
It is not right for your boss to give you shit about what you choose to do with your money. It's bad manners. I'm not the most socially gracious or aware person and I freaking know that. You earned the money, it's not his any more.
Again I'm going to give a counter example to prove my point.
Not saying anything about the large purchase is just one form of boundary to have, and there's nothing wrong with it.
In contrast, I have a very different relationship with my friends and would feel totally comfortable saying "holy fuck, I can't believe you spent that much, to me, that seems insane".
However, anyone who is my friend knows that I come from a place of profound respect. They know that what I mean is "I don't understand why you made that decision, I want to understand you". They know that because it's part of my boundaries that I make that clear to people.
My response isn't better or worse than Morning Glory's, we are just different people who manifest our respect for people in different ways. Anyone who knows either of us well enough would know we come from a place of love and respect.
The point is that there is no correct way to behave, there's just the way that honestly represents you as a good person. As long as you are consistently caring and respectful, the details of how you do that don't really matter at the end of the day.
Yes I piss people off, so does Morning Glory, so does everyone else. It's unavoidable. However, you will always garner the MOST respect that you can in this world if you staunchly live with integrity. And what that looks like in terms of behaviour is very individual.
Your boundaries and how you engage in conflict are how you communicate your level of integrity to the world. High integrity equals very strong boundaries. You cannot have integrity without strong boundaries.