I have been to Canterbury Cathedral many times, and yes, it does cost to get in. There is one obvious exception to the admittance fees-actually attending services (what this magnificent church is intended for). When compared to some of the other CofE treasures (Westminster Abbey, St. Paul's Cathedral in London) here, you get to actually stop and admire the sheer craftsmanship, without being herded around. THAT alone makes this a jewel in the crown.
Some spots not to be missed, the site of the Tomb of St. Thomas-though there is only a candle marking the spot, look closely at the floor, and you can see the indentations from centuries of kneeling on the floor at the shrine-a reminder that you are only one of millions to come to this place. There is also a window that gives an idea of what the shrine actually looked like before being destroyed-it reminds me of the tomb of St. Edward-the Confessor (Westminster Abbey).
The Bell Harry Tower, just look up! Wow.
The grounds of the Cathedral have some interesting ruins to look at.
Make your visit personal, walk around, make the time to take it all in, ask questions of the staff, they have always been helpful, and knowledgeable. Get your money's worth from your visit! Who knows, perhaps this wonderful place will inspire you.