I can’t remember what kind of expense we were discussing earlier this year when my son pointed out how much it would really cost.

“Mom,” he said in the tone 15-year-olds use to explain basic facts to ignorant parents. “Thirty dollars a month would amount to $5,190 after 10 years.”

No, I countered, it would be $3,600.

“You could invest that money if you weren’t spending it and make 7 percent interest, compounded.”

How did he know this?

Plant your own garden to save money on groceries, advises Mr. Money Mustache. (Diana Reese for The Washington Post) Plant your own garden to save money on groceries, advises Mr. Money Mustache. (Diana Reese for The Washington Post)

“Mr. Money Mustache.” My son explained that Mr. MM had retired at the age of 30 and now writes a Web site with advice on how to save money using what I’d call principles of extreme frugality. I can understand the site’s popularity; it’s written in an engaging and down-to-earth (and sometimes earthy) style.

Mr. MM’s advice on how to live without giving into rampant consumerism — my parents called it “keeping up with the Joneses” — does make sense, and, frankly, we’ve followed a lot of his advice over the years, although sometimes by necessity, not choice. Layoffs due to jobs outsourced overseas and subsequent stretches of unemployment for my computer-programmer husband plus my sporadic career in freelance journalism have put our once-firmly-middle-class household alarmingly close to the poverty level where income is concerned.

If you don’t have money coming in, you need to figure out how to have less going out. I think Mr. MM would support much of what we’ve done, although I suspect he would push us to do even more.

Our “newest” vehicle, purchased used almost five years ago, is a 2004 model with nearly 134,000 miles on the clock. In his blog on “short term-itis,” Mr. MM explains how a new Honda Pilot would end up costing you $57,000 more than a nine-year-old Honda Odyssey minivan. I remember years ago when I made the last payment on my Ford Escort, and my boss told me I should replace it with a brand-new BMW. “After all, you’re a senior editor here,” he said, making “big bucks.” I disagreed, knowing that I was planning to launch a freelance career soon and wanted to be free of debt.

Despite our efforts, we’re not at Mr. MM’s level. He advocates bike riding instead of driving whenever possible. We’ve yet to fully embrace that, although my son wants to start biking to school.

Then there are all those trappings that people tell themselves they deserve, like restaurant meals and fancy coffees and trendy new shoes (sigh). We rarely eat out, although that’s one luxury I do miss.

We have cable, though Mr. MM prefers Netflix and keeping his young son away from television commercials, but we don’t subscribe to any premium channels, and I see it as cheap entertainment.

We have “antique” cellphones, according to my kids. They’re the basic models; I’ve refused to pay for phones that send e-mails and surf the Web (although sometimes I want one, too, but then again, do I really want people to be able to reach me anywhere and anytime?) Mr. MM recommends prepaid plans, which I need to investigate.

We haven’t splurged on furniture or clothes. We don’t redecorate on a regular basis, as some of my friends and neighbors do. The couches in our family room are older than our kids. We still have a picture-tube TV … that someone donated to my uncle’s church and the church didn’t want.

When you think about expenses, it’s key to look at the long-term cost, according to Mr. MM. That means multiplying a weekly expense by 752 or a monthly expense by 173 to find out how much that money would have amounted to if invested at 7 percent after 10 years.

That puts a new spin on “it’s only $10 or $20 a month.” And that’s why middle-class people don’t get ahead, Mr. MM believes.

I’ll admit it was a lot easier to embrace frugality when it was just me. It’s hard to deny your kids. There’s always a request for lunch money, a check for a field trip, equipment for a Scout camp-out, tuition for dance lessons…. The list is endless. I feel like Scrooge for saying no.

But as Mr. Money Mustache points out in an interview with The Washington Post, “Realize that happiness comes from accomplishment and personal growth, rather than from luxury products.”

Diana Reese is a journalist in Overland Park, Kan. Follow her on Twitter (it’s free!) at @dianareese.