Some methods cost more than others.
Depositions, for example, can be formidably expensive because they may involve travel and per diem for the prosecutor, defense counsel, defendant, court reporter, interpreter, and an agent. Even the expenses of translating a request for assistance can be substantial. In some countries, local counsel must be retained to present letters rogatory to the court. All such expenses are chargeable to the office that prosecutes the case. Be sure funds are available before making a costly request.