Originally Posted by ORDnHKG

Wrong, paid upgrade is available when the load is low, not when it is overbooked. When the flight is overbooked or more than 75% full, no paid upgrade would be offer at the check in or at the gate.

Just for the information for OP, any US-Japan Y to C paid upgrade is offer for $550, it is prety much standard for any fares, any routes base on my experience.