On the cost note: for travel in some countries you are required to post a bond (at home) relatitive to the value of the bike you are taking. This is to ensure that you are well motivated to take your bike with you when you leave the country in question, and do not sell it. Having a new bike with a high book value can present a serious cash tie up. There is also a bit of comfort in a lower 'walk away' factor I think. One could spend a lot of time and energy extracting a crashed or broken $25,000 invested-my-life-savings machine out of some remote place, rather then being able to leave it and cut your loses. (but let's not focus to much on that negative thought!)
Also, there is always the issue of looking like you have lots of money, which a shiny new machine certainly presents.
In the BMW world of things people often seem to be trying to make the choice between the new 1150 GS and an older R100GS or R80G/S. Don't forget about the F650 series as well, which are closer to the older Airheads in size, weight, and power.
Personally, the lighter weight and simplicity of the older series does attract me. Although in many ways I have come to realize that the R80/100 GS series strikes an awkward compromise: it's capabilities in the dirt do not extend far beyond the 1100/1150 GS to justify what it gives up in brakes, ride quality, and comfort on paved roads. If dirt riding is a big part of the plan, then a lighter single such as a KLR or F650 dakar would be better still.