Yeah it sucks. I managed to join while a deal was on so it was a bit cheaper, but I've joined before and the price has really made me wonder whether it was worth it. They should definitely let you join on a monthly basis.
I wonder if they make you commit to 3 months because after you join a lot of the sheets are downloadable so expect a lot of people would only sign up for a month to get the information and buy the books from the store. If I can afford it I like to sign up for online but when money's tight (at the moment) I try to go alone with the sheets I've downloaded from online.