I think when I checked this on their website it worked out at about £20 a month because I remember thinking it was about the same for 4 weeks of classes. I think I found it on their FAQs section under how much does it cost, I remember it took me a while to find it, they only seem to tell you the price for the first 3 months.
I ended up joing a class as I thought it was expensive online, and I might as well go to a class anyway. I don't think you get the books either, you have to look everything up on the site.
When I did WW online it was £10 a month after the initial startup price, I would have expected SW online to be cheaper than going to a class too!
Although if you can't get to a class I suppose its another option.