Sold R99 membership, being charged more than double!

In September, my husband and I went to the Virgin Active Lorraine (Port Elizabeth) to query prices on signing up at their club. The sales lady sat with both my husband and me, and convinced us to sign up - selling us on their current promotion (their best promotion yet!) She said that for September-November, we'd get to gym for free and then from there on, because we are off-peak members, our membership would only be R99/month. This seemed like a crazy good deal, and given that she said we'd have to sign up by the end of that week to take advantage of the deal - we didn't want to delay. I also signed up my child to the kiddies area at R105/month. So, in total, my account should be R303 each month. However, I am being charged R709 - more than double the amount that was discussed and communicated to me. I have contacted the club twice, and contacted head office who say, "sorry, we don't have a membership like that." Today, the club said, "She probably didn't discuss the membership fees properly and might have gotten confused - R99 is the joining fee, but you're liable to paying R305. (***!)" Unfortunately, her mess up is costing me a whole lot of money and head office is doing nothing to help me. I cannot afford more than double what was explained to me because of an *********** sales rep - surely, I have the right to cancel the account due to their negligence???