As part of our program’s 25th anniversary, we’ve been tracking the odd ways prices have changed over that period. According to a cellular phone industry group, the average monthly cellphone bill has dropped from an inflation-adjusted $151  in 1989 to $47 today.

A 69 percent drop? How is that possible? 

Michael Grubb, an economics professor at Boston College who has studied the cellphone market, shares reasons for this downward trend.

Listen to the full conversation in the audio player above.