Just a dumb joke about how not to waste money.
Like, if you were going by how many measures you listened to, instead of time (or like the one person said, wearing an item of clothing at least once for every dollar spent), then you'd reach x-many measures faster on songs with higher bpm's. So if you really enjoyed a pair of headphones, you'd want to listen only to slow music, so as to prolong the time you had with them. And with crappy headphones, you'd want to listen to fast music to get the x-many required measures over with quickly, so you could get rid of the headphones without feeling guilty for not having used them. But then if you actually preferred fast music generally, like blast-beat metal, you'd be screwed because your x-many measures would always be over so quickly. Ha ha ha... Get it?
OK, so it's a really dumb joke about how not to waste your money...