Welcome to the forum.
You could buy a Network Railcard for £30 which would give you a 34% discount on the price of the tickets, making each single journey £11.35, £22.70 for the return.
Alternatively you can buy off peak carnet tickets - £57 for 5 single journeys (you'd need to purchase a London-Cambridge set and a Cambridge-London set) so costing £104 for 5 return journeys working out at £20.80 for a return journey. Carnet tickets are valid for 3 months from date of issue. Unfortunately you can't get a railcard discount on them.
If you do opt for carnet tickets make sure you date each one correctly - we've had numerous threads on here about people re-using these tickets, and having difficulty writing the date onto the tickets due to the quality of the ticket paper - don't get caught out!