assuming your buying something that has no value to you just for the miles (through your carriers "store"), at what point is it a no brainer for the purchase. if you value your miles at 1.X cents per mile then obviously its less that but, how low do look for before you load up on miles, er I mean purchases?

Came across a deal that lets you "buy miles" at 1.6 cents per mile and I dont see the value of it at that price but, was curious what others see when they look at miles purchases