Everyone values miles differently - there is no hard rule. There is a general rule, but to me, they are worth more. I have never paid to travel in a premium cabin, but due to the miles, I have sat up front quite often, whether upgrade or free ticket. A 90,000 J award to Europe, at 1.5 cents, comes to $1350. I can't buy a J ticket to Europe for that amount (at least when and where I want to go), so in this case, the miles are worth much more, to me.

YMMV.