Slightly better is Keighley - Wakefield Westgate Anytime Day Single at £4.80 and
Wakefield Westgate - Sheffield Anytime Day Single at £8.70 NOT Doncaster
The more you can get within the WYPTE boundary, the better, since they are quite subsidised fares. If you can go from Keighley, you will save something over going from Steeton (which I expected to return a more similar fare!)
Couple that with the £12 SHF - NOT gives a total of £25.50
To be honest, doing direct splits at stations called at, I think you will be hard pressed to get anything better, as the XC to Sheffield is non-stop from Wakefield and the Sheffield - Nottingham is fairly limited stops.
A saving of over £15 over the SDS for the whole journey is pretty good I think!
EDIT: OK, make that £18...good going Indigo2!