I had a TXL-PEK this week. I asked about upgrades during check-in. The person offered to check if there was space available, but suggested for me to know the price before she goes into the system. I agreed, and then she got a small paper in front of her monitor, checked my booking class, and gave the amount: for V fare class it was 1100 or 1300 EUR. I could see about 5 different groups of fare. I asked if I could take a picture of the paper but she didn't allow me. And I didn't get the upgrade.

To my experience they are available and there is a standard price card.