The most convenient way is taking flights. From Nanjing to Katmandu, there are only conjoint flights with transfers at Kunming, Guangzhou and Hong Kong available. As for the cost, it is hard to say since the airfare for the international flights changes from day to day. Thus, you would better check the ticket price with your exact travel date. Besides, it would be cheaper for you if transferring at Kunming or Guangzhou.