It's just a few pence less than the Sheffield/Dronfield to York fare.
However there is no CDR for Rotherham so it's cheaper to buy from Dronfield/Sheffield and start 'short' at Rotherham off-peak!
If returning another day the fare from Rotherham to York is cheaper than from Doncaster to York. This is due to EC (and NX before them) ripping us off with higher fare rises than Northern. Returning same day it is cheaper to split at Doncaster but only if travelling off peak (£4.30 RMC-DON CDR after 0930 + £11.70 DON-YRK CDR after 0915) totalling £16, a saving of 30p compared to the Sheffield/Dronfield to York fare.
In the unlikely event of the times of the Northern trains suiting, Rotherham-Sherburn £9.90 SDR, Sherburn-York £4.40 SDR, total £14.30. Not really worth it to save £2.50 though IMO.
Anyway the best ticket to get for same-day off peak travel is Shireoaks to York CDR at only £14.70. This is valid via Sheffield as it is actually shorter via Sheffield than via Retford, however the price is set by EC who assume travel via Retford and charge less than Northern for off-peak travel, but charge considerably more for peak travel (the SOR from Shireoaks is a staggering £38.60!). I always buy a Shireoaks ticket if travelling to Sheffield off-peak returning the same day.
Shireoaks and Kiveton park are
near each other but a long way apart when it comes to fares. This is due to differing policies applied since privatisation. And the gap is getting bigger all the time.