I trekked for gorillas (the Amahoro group) in Rwanda on May 26 this year, and it was one of the highlights of my travels. In answer to your questions, we hiked for approximately three hours (in addition to another one-two hours following their movements) and struck some very tough terrain (in my opinion) but we were told afterwards that this was a normal trek. We managed to see 8-10 gorillas in total - including babies, young ones, greybacks and silverbacks. The cost of the permit was US$500.00 - which I believe is standardised across the three gorilla countries (Uganda, Rwanda and DR Congo).
However, there are some cautions:
Stinging Plants - the bane of my existence - I even had gloves and thick socks, but they still had an impact. I wish I had worn thicker pants. Not sure of any way around this, possibly an anti-sting spray will numb the area, but not sure if it is necessary since the pain only lasts a few minutes.
Terrain - there can be some very steep sections, including vertical climbs and descents, and this isn't helped by the moisture that swirls through the air and causes the ground to be squishy and slippery. You may also experience light rain, so bring a light waterproof jacket. You may not strike such challenges, but be mentally prepared just in case.
Altitude - we were over 3,000 metres, and the thinner oxygen has a stark impact on your endurance. I was lucky to have hiked through the Simien Mountains in Ethiopia a couple of weeks before the gorilla trek, and this helped me tremendously. Also, higher altitudes can increase your water consumption - so bring plenty.
Surprise, surprise! - One gorilla unexpectedly emerged from the undergrowth and walked right through the middle of our group - his fur literally brushed my leg as he sauntered past, and he grabbed another party member to shove him out of the way. These are wild animals and anything can happen, so follow the advice of your guides without question.
Overall, was it worth it - absolutely yes!!! My blog of this day, Gorillas: A Very Close Encounter
, will give you the full story.