Data point:
SIN-HKG-KMG, original fare bucket V, upgrade options as follows:
SIN-HKG, V -> E, min. SGD245/USD181, max. SGD555/USD409
HKG-KMG, V -> I, min. SGD370/USD273, max. SGD970/USD715, plus SGD16/USD12 extra in tax
Meaning a combined minimum of USD466 and USD1,136, in other words, not that exciting...
If the SIN-HKG leg was available for upgrade to I at the same price, I might consider putting in the minimum bid, but as that's not an option I am going to pass on this one...