Making some assumptions as to what qualifies, I think the "best" I can come up with is York - Derby day return departing at 0826 or 0845. Cross Country want you to pay £89.50 for an Anytime Return (there is no Anytime Day Return)
However all trains call at Sheffield, so you get a York - Sheffield Anytime Day Return £19.80 and a Sheffield - Derby off-peak Day Return £10.80 totalling £30.60. Valid any train on the way back.
This reduces the price to around 1/3 of the fare with a simple split.
Thanks for this info, Yorkie.
I kept this info in mind and I am travelling one way from York to Derby at the end of the month.
While the saving is not as big, I am purchasing a York to Sheffield Anytime Day Single for £17.70 and a Sheffield-Derby Off-Peak Day Single for £10.70 - a saving of £12.20 on the Off-Peak Day Single price of £40.60.
(I did not want to buy an advance ticket as while I know the day I am unsure of which train I will catch on the day).