Its a good idea if someone is willing to sell you a pound of dank for 300 dollars and you only have 5 dollars. You flip 2 ounces and you've already paid it back, profited and have 14 ounces left.

Obviously that would almost never happen though, but it doesn't have to be pound, someone might have a really good deal for you to where losing $75 is worth it because you'll end up profiting in the end.