The best savings for this journey are likely to be made by buying Advance tickets, normally splitting tickets at Peterborough, and/but sometimes also/instead splitting at Grantham or Newark Northgate.
If Advance tickets are not an option, and if you can leave Peterborough after 0930, I agree with the split at Peterborough suggested by Indigo2.
If Advance tickets are not an option and you have to travel earlier, the price to beat is the Cambridge-York £113 anytime return. One set of tickets that beats that is the following:
Cambridge-Peterborough £19.20 anytime day return
Peterborough-Grantham £24.20 anytime day return
Grantham-Newark £9 anytime day return
Newark-Doncaster £23.70 anytime day return
Doncaster-York £19 anytime day return
Total: £95.10
However ... the difficulty will be finding a journey with stops at all of those stations. But outbound you will be ok on the 0706 or 0759 from Peterborough, and returning you will be ok on e.g. the 1203, 1402, 1602 or 1802 .... from York.