As per their website - http://www.istana.gov.sg/IstanaGrounds/OpenHouse/
As lifted from the website.Admission to the Istana grounds is free for Singaporeans and Singapore Permanent Residents. Others, including foreign visitors, are required to pay an entrance fee of $1 per person. Entrance to the grounds is via the main gate at Orchard Road. Opening hours are usually from 8.30 am to 6.00 pm.