If you must go in January, it will be expensive: it's the peak of their high season. If you are willing to go in December or from February they are offering three days for around USD850 on their website
http://www.futaleufuexplore.com/rafting.htm
White water rafting will never be cheap when it depends on the same highly experienced, highly qualified raft guides and safety experts who move from place to place according to the season. This year my son, Elhijosinnombre, (a highly experienced, highly qualified raft guide and safety expert, if you hadn't guessed already) has been working in Uganda, Austria and Canada and in a few days time will be joining River People Rafting in Tena, Ecuador. It's what they do.
If you are looking for the experience on the cheap, you could always throw yourself into the water with a plastic barrel for flotation like they do at Bujigali Falls. But on a Grade IV river, I don't recommend it.