If you have an open travel plan, like 12+ hrs of transit time and 2 stopovers, Chinese airlines are the best choice (around $500 return trip). But it is not for those who want to reach Nepal ASAP and for those whom money is not a problem. Another plus point taking the Chinese airlines is that they provide free hotel accommodation, breakfast, and airport transfers (if your transit is more than 12 hrs, but they provide this facility for one way only, not both ways). Please note that their fare may be higher for peak season (like summer).