This morning I travelled from Doncaster to King's Cross and got one for £35, which is just over what I paid for my original ticket (£31.20)! When I checked availability yesterday it said starting bid £25 or I could bypass the auction and "buy it now" for £35 so I did that. The busier the train, the higher the starting price, I would guess.
I think LNER Weekend First is £25 from Doncaster to London. Upgrading with the TM when on the train would save a tenner.