Cultural Triangle Ticket
Polonnaruwa is included in the Cultural triangle Ticket which represents a good value to get rather than paying for each site separately.
This cost around 40$ (!!) and covers six sights (Anuradhapura, Sigirirya, Polonnaruwa, Nalanda, Medirigiriya, Ritigala BUT NOT Dambulla caves and certainly NOT the temple of the tooth in Kandy.
The ticket is valid for 14 days from the date of first use and within 3 months of purchase. Unfortunately (again) it is valid for a single and NOT multiple entries in one sight.