More expensive; requiring greater financial expense than something else; the comparative form of costly.
From costly (cost + -ly, meaning characterized by) with comparative -er. The word naturally formed as English developed ways to compare prices and expenses.
Interestingly, we say 'more expensive' in modern English but 'costlier' still sounds more dignified—which is why jewelry ads and luxury brands use 'costlier' to suggest refined taste rather than mere high price.
Complete word intelligence in one call. Free tier — 50 lookups/day.