Hey Talha Siliguri to mangan ... the cost of a bus ticket would be around 1300 to 1500 and would take around 6 hrs to reach .. From mangan you don't get many buses and it's better you take a taxi to lachen which is around 60 Kms ... so you need to book a cab with a round trip from mangan where he takes you also to Naga waterfalls ... Happy Travelling Bro!!!