The most boring way: by train. Virtually nothing to see. But at least it's fast and cheap compared to buses. They're probably that expensive because they're all sleepers (just guessing here). The much longer travel time is explained because they go across and around the mountains, while the train goes straight through the tunnels, the main reason why you hardly see anything of the spectacular scenery.
If you're going the same way I think you should double-check for cheaper buses and/or maybe stay somewhere overnight while on route.
Do check out my "Southern Silk Road" thread with some updated info about Yining if you plan to go there.
drftr