A lot of people seem to book tours ahead of time but i suggest sorting tours when you're there since you normally need only 2 days to organize everything with most tour operators like Tseren Tours, who charge $130 per day for 2 persons in a jeep with guide and driver all in.
Couldn't agree more! I'd to skip booking online and paying over the odds when you can easily and cheaply hire a local guide there (most of them speak english) for around $30 a day. Another plus is that you'll be supporting the locals.
when traveling to mongolia, I suggest that you explore on your own part of the time your're going to be there, and the other time to go with a tour operator like Hovsgol Travel or Mongol Global. Nadaam, the Gobi, Hovsgol, and Terelj are all beautiful and a must when in mongolia. Ulaan Bataar, the capital, is easy to get around by the way.