My employer only covers travel in Y. However, I am can bid/upgrade the fare to C at my own cost.
On my bimonthly POZ-WAW-LHR travels, I book the flights in Y and do plusgrade bids to C mainly so I can take two pieces of luggage on board.
I do not bid more than 100 GBP on the flights (got some in the past for 75 GBP but inflation even hit plusgrade bids and nothing on my route is less than 90GBP).
My logic is that 100 GBP would be the average cost of two lounge visits, extra luggage and security fast track.