using NFM95
London Terminals - Hull is £75.50 for an "Any Permitted" Saver Return, the "Hull Trains Only" Saver Single is £36.50 (equating to £73 return, so pointless if you only go one way).
London Terminals - Doncaster is £63.80 for an "Any Permitted" Saver Return, the "Hull Trains Only" Saver Single is £30.50 (equating to £62 return, equally pointless)
There is only really one benefit to Hull Trains: they will sell discounted tickets, such as Savers, on board the train with railcard discounts when the ticket office is open (you can see this on departure boards as "TICKETS FOR SALE ON HULL TRAINS" - of course, they are also sold on GNER but only at a premium (Standard Open/Day and no railcard discounts) and that's the key difference).
The other day I was (unusually) on a HT service, and was talking to a young lady who had a Y-P discounted Saver Single, she was having a 'debate' with the conductor about a reservation she thought she had - but didn't. Anyway, she had no idea that the "Any Permitted" return was about the same price as the singles she was buying and gave her much more flexibility. She won't be buying HT-only tickets again...