Yes, it's free of cost.
On Weekdays (Monday–Friday) - 08:00–15:30, 19:00–00:00 & On Weekends (Saturday–Sunday) - 05:00–15:30, 19:00–00:00.
On Weekdays (Monday–Friday) - 15:00–18:30 & On Weekends (Saturday–Sunday) - 15:00–18:30.
There are several options including; At the Top, Burj Khalifa (Level 125+124), At the Top, Burj Khalifa SKY (Level 148+125+124), World’s highest lounge (Level 154+153+152), etc.