It's both the amount and what it can buy, and you should be lecturing your students that way.
"Growing up in the 60s the min. wage was $1.60. Gas was .23 per gallon etc. I have low six figure income now. I drive a 1999 van. The money is worthless."
So in the 60's, you could afford 6.95 gallons for every hour you worked.
Now you make at least $100,000, which is equal to $50/hour and at today's price of $3.50/gallon, you can afford 14 gallons for every hour you worked.
You're making a lot more gallons per hour than you were in the 60's, so a low level of inflation over 50 years has not stopped economic progress.