Your tour operator or hotel should give you a complete list of available tours with prices... it is usually less than 100USD per person...It depends on some factors which will be explained to you be those offering the tour. You may perhaps save a little money by buying from outsiders, but the difference may not be significant and in the end you do run the risk of falling prey to some scam...Unless you are SURE you know what you are doing, buy from your tour operator or hotel.