Nowadays plaster ceiling 4 inches enough already
If you really wanna save then 3 inches

If your ceiling is quite nice from developer then you could consider to skip plaster ceiling in the middle, then the sides do light trough / plaster ceiling with downlights

and like what someone else said, LED downlights are very slim nowadays, around 2 inches only

and on the costing, a flat plaster ceiling goes around RM 6.50 per  sq ft, excluding the light trough which i think is RM 20 per foot run