I would consider making it a game.

"Let's compare the real cost of both phones. The phone that gives you the most flexibility and resilience wins."

"So here's the cost of the iPhone 7 Plus, monthly, in this spreadsheet cell. So let's set up some formulas to see what that cost looks like annually."

"And here's the cost of the iPhone XS, monthly, in this other cell. We can copy and paste the formulas for the other phone -- look, cool, now we can see the annual cost."

"Okay, here's the difference between the two columns. That's the savings you would have if you had the iPhone 7 instead. Now if you invested that at an average 7% return -- and here are the formulas for those -- let's see what it looks like in 5 years, 10 years, 20 years. So in 10 years, you could have either a phone and 0 dollars, or a phone and X dollars. You could use that X dollars to buy so many things, which gives you flexibility. If you were in a job you hated, you could even use it to let you quit that job because you wouldn't be living paycheck to paycheck. Or if you got laid off, you could use it to make sure you don't lose your home. Those give you resilience. So which lets you win more flexibility and resilience, the iPhone 7 or the iPhone XS? You can choose either. This just helps you understand where you win and where you lose."

"And btw, you can do this same kind of comparison with lots of other things. Salaries, car payments, mortgages, investments, etc."

When I was in high school, we actually learned a bit about finances: How to write a check and balance a checkbook, how to pretend invest in the stock market (we picked a few stocks in the newspaper and followed them for a week, I think). I'm sure I brushed it off at the time, but that stuff stuck with me. The magic of compound interest really can be very alluring when it looks to a young person like money can grow and grow.