I am very new here to the UA Forum. I just want to reply to your post to help keep it current.
I checked in at the kiosk in Houston for my Houston SFO Beijing flight : the computer asked me if I want to upgrade for Y+ to C, for $919 ( I don't why that figure ! ). So, I did.
On another trip 3 weeks from that, I was in C class, and heard an onboard announcement that if the folks in C wanted to upgrade into F, it's $700.
I went to the manager looking guy and asked about upgrade from Y+/Y to C :
he told me that it's all depends on the load in YC and C. If they have a lot of seats left in C, they would price the paid upgrades lower.
Was my $919 upgrade from SFO PEK at the high end or the low end, I don't know. Perhaps some of the old hands here at this forum can help us out with the infos.
Welcome to the UA forum. ^