Here's the deal for London to Cambridge via train.
Weekdays after 9:30 (the 09:31 is valid) and all day weekends (watch out for scheduled engineering works on the weekends) London Kings Cross (KX) and London St Pancras (STP) have departures on Great Northern (KX) and Thameslink (STP) which take as little as 49 minutes (KX) and 1:05 (STP) interchangeably using an Off-Peak Return (what you might previously have heard called a Cheap Day Return - the person behind the window will know the term, the machine and online won't) for £27.60. Again, M-F after 9:30 northbound, any time Sat, Sun, valid on the return at any time after M-F 9:30, so no evening restrictions. The only restriction is the 9:30 one in the northbound direction. The 49 minute trains from KX are nonstop as far as Cambridge.
By interchangeability I mean you can go on Great Northern and return on Thameslink or vice versa, the same ticket valid on both regardless of train colour or KX/STP. These trains do not offer "Advance" tickets.
Same time restrictions at Liverpool Street for trains which take 1:11 but most are 1:24. Off Peak return is £19.50 First train valid is actually the 9:28 which takes 1:24. However non-changeable Advance tickets as late as the day before are available for £12 single during the peak, £10 in the near peak and £8 after 10:15 (10:28 the first), and for the return into the evening rush £8, £10 or a few £12 depending on capacity, availability and time (and of course what they think people will pay). So if you work it right and plan ahead you can go on the slow train for as little as 8+8=£16 for the day.
As always, read and check, check, check; and if using Advance tickets you MUST travel on the exact service you have bought.
Hope that helps.