The aux cable kit is about £20 from Ford and the number is 1426121.
Alternatively buy off eBay for the same price-http://pages.ebay.com/link/?nav=item.view&id=171108283181
To be honest the standard fittings are not up to much, too much slop on the socket which causes crossed connections and poor sound.
I use one of these with the wire coming out in the cubbyhole under the demister switches.http://pages.ebay.com/link/?nav=item.view&id=350828164832
No drilling of holes needed and better sound quality.
All you need to do is plug the connector block into the quadlock connector plugged into the rear of the radio.
You need stereo removal keys and although you can plug the connector in without disconnecting the radio, i do recommend you have your radio code handy.
What they were quoting you for was one of these-http://pages.ebay.com/link/?nav=item.view&id=251224717949
Handy if you want your steering controls to be able to skip tracks on an ipod etc but no track listings come up on the stereo screen.
If you need to work out how to pull your stereo out I have a guide for swapping radios under this messsge which describes how to do it in the first section.