A flight from Rupsi to Guwahati Borjhar, which has 0 stops, is S$ 42 which is the least expensive round-trip domestic flight that flybig offers. For a one-way domestic flybig flight, the cheapest route is Itānagar Hollongi to Guwahati Borjhar (0 stops) for S$ 22.
flybig does not have flights from Singapore Changi at the moment. There are scheduled flights with flybig out of Imphal though (if you don’t mind flying from this airport). Flying with Alliance Air is recommended if you still want to fly from Singapore Changi.