I haven't seen any prices since Covid, but prior to that the agents at check-in had a sheet listing the prices for the day for each flight.
The consensus seemed to be that there were rarely bargains, and very few people seemed to upgrade with cash at check in.
It will also depend on what type of economy ticket you already have.
To get an idea, if you have booked with VS, you can look at the 'bid for upgrade' option on the VS website - not sure how close the minimum bid is to the price the airline consider acceptable though.

Nick