There's no need to go all the way into London for this journey. I would change at Peterborough and Ely.
Booking on East Coast's website, current prices:
Advance Doncaster - Peterborough 09:20 departure £7.10
Advance Peterborough - Doncaster 19:23 departure £10.20
Walk-up Off-Peak Day Return Peterborough - Cambridge, route Via Ely £16.60
Total: £33.90
You should of course check that these times match up and you don't have a long wait at Peterborough. Otherwise, there are other APs that are slightly more expensive.
The problem with Grand Central and Hull Trains is that they don't call at Peterborough, so you'd have to go via London with them which would drive up the price
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There is also a ticket you could consider, Off-Peak Day Return Peterborough to Cambridge Route Via Hun Busway, at £11.30
I believe this means taking the train to Huntingdon and the
guided busway between Huntingdon and Cambridge. It's
timetable (Route B) is a bit slim on Sundays though.
Could be interesting, hopefully someone who knows more about it can comment on whether its worth the £5.30 saving
