[Scaevolus]

The overhead is slightly worse when it's compressed alongside normal HTML content (the Huffman trees aren't so favorable).

    $ wget http://en.wikipedia.org/wiki/ASCII
    $ (cat ASCII;dd if=/dev/urandom bs=8000 count=1 | base64) | gzip | wc -c
    50578
    $ (cat ASCII;dd if=/dev/urandom bs=9000 count=1 | base64) | gzip | wc -c
    51621

Around 4% overhead. Either way, it's negligible.