Good grief, how often are you going to Tibet? You want to come back with a bunch of bad photos because, sniff, you are so poor (going to Tibet)(?).
You will not miss the money after a while, but you will feel terrible forever if you miss good shots. No regrets, no time to be on a budget.
You need the 18-200 VR zoom and a wide angle. The VR part is important what with all the twilight mountain shadows and all. That zoom is a Nikon-only product and they make that super wide zoom as well, but you could use an aftermarket product. Lots of stuff in Tibet is far away, so a long zoom is good. The wide ange would be great for street scenes, village shots, interiors, people, etc. rather than panoramas of mountains. You have seen those shots where everything is small and far away. Not usually so great. But, what the heck, digital is free and you can toss stuff out.
With those lenses you would not need a tripod and that will be very welcome. You will also have a minimum amount of equipment to attract attention.
Why bother with a 50mm? Some mostly mythical better quality? No, you need to get the shots! Rent those lenses, get a loan and sell them when you get back, whatever, and get insurance! You could sell your equipment on ebay in about 10 minutes, before the credit car bill even comes!
Years later you will not miss the money, but if you don't get what you need you will have to explain your pile of bad photos with tiresome poor stories that nobody will want to hear. Sleep on piles of yak dung, eat entrails-whatever, but do this right.
You also need extra storage in the form of cards, ipods or something, and at least one extra battery. And the recommended polorizer. Investigate what kind of electrical power is available, or not, and get what you need to recharge your stuff.
Lastly, learn how to use your stuff now so that you get all your equipment has to offer. There are landscape modes and such that may be of help. There are also people that suggest different default settings to get more saturation and that may be good at altitude. Check out Ken rockwell's site for that.
Do this right!