Cost can be recovered easily by toll within 30 years.If this tunnel is built, then trade between BD and Nepal will skyrocket.Nepal is heavily import dependent.They imported goods 10 billion dollar last year and increasing fast.Our export there will increase many folds.Plus transit related income. Albeit at the expense of India.Currently India enjoy near monopoly in Nepalese market.Biggest hurdle will be persuading the benya to allow a tunnel across Siliguri corridor.I have no idea on how Nepal will achieve that.India will not allow anything which will reduce dependency of Nepal upon it.