2 for 1 voucher will get you in for the first day. then you can get a bounce back ticketsl for the following say which I believe is £10 for the next day or £15 for any other day.
They're £9.50 for the next day, not sure about the price for the any day bounce-back tickets. I've never had to sue them but they sound like very good value.
Also, Kellog's are still doing the Adults Go Free promotion, which I've used to get family/friends who don't have a MAP in to Towers for the day.