pushing 'http://www.theguardian.com' out for every link
    adds to a lot of bytes up for a busy site

Fewer than you'd think after gzip compression:

    $ curl -s http://www.theguardian.com/us | wc -c
    223195
    $ curl -s http://www.theguardian.com/us | \
       sed s'~http://www.theguardian.com~~' | wc -c
    215473
    $ curl -s http://www.theguardian.com/us | \
       gzip | wc -c
    33783
    $ curl -s http://www.theguardian.com/us | \
       sed s'~http://www.theguardian.com~~' | gzip | wc -c
    33554

They have 7.7k of extra html due to repeating "http://www.theguardian.com" for every link, but gzip compressed this is only a difference of 229 bytes.