online they are quite cheap. I got mine for about £65, although you can get them cheaper depending on style and what size each online shop have in stock. Mine took about 2 weeks to arrive from the states I think.
First thing to do is to go into Lewis's and try some on, see if you really like them and what size you actually are in them, as they are not quite the same as you would be normally.
The MBT's I bought about 1 month ago were £169 in John Lewis, so I "saved" over £100. They are exactly the same except a lighter shade of brown.