costly
adj.adj. expensive or causing a lot of trouble. You use this when something costs a lot of money or when a mistake leads to a big loss.
adj. entailing a high price or significant expenditure; resulting in great loss or sacrifice. Often used figuratively to describe errors or delays that have serious negative consequences.
The repairs to the old car were very costly.
A costly mistake in the final minutes of the game led to the team's defeat.
The project was eventually abandoned after a series of costly delays made the original budget impossible to maintain.
From Middle English costly, costely, costeley, equivalent to cost + -ly.
Typically used as an attributive adjective before a noun, though it can also follow a linking verb.
He ran costly to the finishHe ran at a high cost to himselfDespite the -ly ending, costly is an adjective, not an adverb; it cannot modify a verb.