Be aware that the early booking discounts are no longer available for many of the dates (everything from June through almost the end of August), so you won't be able to take advantage of that on some of the trips. Also you will need to figure in tips for the guides, and the cost of several meals which are not covered by the tour. You can get the exact numbers on the website.
Also note that if you have an annual pass, you get 10% off.
I just booked our tour a few days ago, and while I am still in shock over the price I agreed to pay (I thought I could get the early booking discount but couldn't) I am so very, very excited about the trip!