the current fee is £72.50 for a standard 32- page passport.
http://www.direct.gov.uk/en/TravelAndTr ... /DG_174100
Then, if you wish the IPS to return your other supporting docs via secure post, you add £3 and, finally, if you want to use the Check & Send service at a Post Office (thoroughly recommended) it is an extra £8-£9 and they send it off using their own special couriers (never seen/heard them using special delivery, they put your application into a transparent bag, seal and post it off).
Altogether, around £85. Good luck.