Collocations for costly - adj.

be, become, prove

extremely, fairly, very, etc.
enormously
especially, particularly
increasingly
potentially
prohibitively
relatively
politically
The candidate's mistake proved embarrassing and politically costly.

for
The six-month delay will be costly for the company.

in
This process is costly in computer time.

in terms of
These teaching methods are too costly in terms of staff resources.

to
These measures could be costly to employers.

Costly is used with these nouns:
delay, disaster, exercise, failure, litigation, mistake, repair, undertaking, venture