IPA: /ˈkɔːstliər/
KK: /ˈkɔstliər/
More expensive than something else.
The new smartphone is costlier than the previous model.
Costlier → It is formed from "cost" (from Old French *coste*, meaning expense or price) and the suffix "-lier" (a comparative suffix indicating more). The word "costlier" means having a higher expense or price than something else.
Think of the word 'cost' which relates to expense, and remember that 'costlier' means something that has a higher expense.