As for how much you lose.. well how long is a piece of string? It depends if you stick to it 100%, how much you have to lose and lots of other things. but 1-2lb a week is about average.
I've been working at the pay and weigh since April and in all that time its never been full price. They pretty much constantly have an offer on - either its half price membership plus your first week's meeting (so £9.95 on your first night) or its full price joining fee but your first meeting free (so £10)
I think you'd be pretty unlucky to join in a week where there are no offers on - I've not yet seen it happen