We are planning to take a day trip from London to either Cambridge or Oxford in a couple of weeks. I was looking at train ticket prices on the First Great Western site and found that the tickets to Cambridge cost about 3x more (£7.5 vs. £21)than similar tickets to Oxford. Is that normal or did I do something wrong?
If you specifically choose to travel on National Express East Anglia trains from Liverpool Street to Cambridge, you can get an off peak same day return (roundtrip) ticket for 15.80 GBP. For other train companies the fare you found is correct.
Besides the train, you can also travel to Oxford on the Oxford Tube (highway bus). The return (roundtrip) fare is 16 GBP.