I have found 1 for £37 which get you in at at the same time as leaving on the 8:15 via London
Manchester Piccadilly (MAN) to Leeds (LDS)
Journey Summary Details - Option 1
Departs Arrives Duration
08:26 09:22 0:56
Thu 17 Dec
Leeds (LDS) to Peterborough (PBO)
Journey Summary Details - Option 2
Departs Arrives Duration
09:40 11:03 1:23
Thu 17 Dec
Peterborough (PBO) to Cambridge (CBG)
Journey Summary Details - Option 3
Departs Arrives Duration
11:16 12:08 0:52
The very cheapest I have found 1s £18:50 splliting at Leeds on an Advance ticket but does not get to Cambridge till 12:44 which may be too late for you.
Thu 17 Dec
Manchester Piccadilly (MAN) to Leeds (LDS) £6
Journey Summary Details - Option 1 Departs Arrives Duration
08:42 09:36 0:54
Thu 17 Dec
Leeds (LDS) to Peterborough (PBO) (£12:50)
Journey Summary Details - Option 1
Departs Arrives Duration
09:05 10:35 1:30
Thu 17 Dec
Peterborough (PBO) to Ely (ELY)
Thu 17 Dec
Peterborough (PBO) to Ely (ELY)
Journey Summary Details - Option 2 Departs Arrives Duration
11:45 12:22 0:37
Thu 17 Dec
Ely (ELY) to Cambridge (CBG)
Journey Summary Details - Option 3 Departs Arrives Duration
12:26 12:44 0:18
As for coming back at that time £37:50 is the best via London so far and I guess the one you have already found.
If you use the NXEC website to book the tickets, you can get to Cambridge for £11.15 on one of their NXEC advance discounts. But that's the same train which arrives in at Cambridge @ 1244 which may be too late.. Other than that, they seem to be around £32.95 from NXEC at the earliest of 0705, arriving in to Cambridge at 11:08. Those are all the ones changing at Peterborough.
As for the return, I've not been able to find anything cheaper..