Yes, Ticket is mandatory for visitor in the heritage site. The ticket price are as follows-
Swoyambhunath Stupa-NRS 500/-
Boudhanath Stupa-NRS 500/-
Patan Durbar Square-NRS 1500/-
Bhaktapur Durbar Square-NRS 1500/-
Pasupatinath Temple- NRS 1500/-
SAARC countries have different prices.