The rough distance between Cambridge and Oxford is 84 miles.
The trip on a bus takes around 2 hours.
The price of that bus ticket through Megabus.co.uk can be found for 3 Pounds Sterling!
Thats LESS than 5 U.S. Dollars! (I pay 5$ most days to commute to and from work.)
Megabus is an insanely cool bus system that runs throughout the UK. You have to be clever about booking; the more advance the more likely you are to be able to grab a seat so inexpensively. Also check your city because only certain cities go to certain places.
The ONLY down size and my recommendation: USE the restroom BEFORE you get on the bus and not for the usual reasons one might think of in avoiding public restrooms. But rather I must tell you a technical truth here – the Toilets are made for people with no more than a size 6 MAYBE a size 8 hiney! Seriously, when they were making buses in the 80’s-90’s the average British behind must have been VERY small. That being suggested = GO MegaBUS!