Front or rear?
The rear is about £60+vat from vagparts, i think the front is around the same price but there's nothing on their webpage - give them a call though!
I've never had to replace any so not 100%!
Is the motor definately gone? If it is the rear, there is a quick fix guide you can do. someone (i think HTC) posted a 'how to' a while back. If you do the search you should find it.