I had a friend who always used to keep a small tub of baking soda beside her toothbrush.....when she brushed her teeth, she would apply the toothpaste to the brush and then dip it in the baking soda. She swore that not only was it much cheaper but that it was just as effective.
You can buy this toothpaste in Boots or any larger chemist or Health Shop. One of the most popular brands is called 'Arm and Hammer baking Soda Toothpaste' and is also available in a gel form. It's not cheap so it might be an idea to go the way of katejess' friend and see if you like it first. Definitely an aquired taste - people either love it or hate it.
I agree with katejess. My granny used to say exactly the same thing, although i dont know whether she used baking soda on a regular basis. Sometimes its best to listen to the old huh!