The rule of a thumb is that a "bad hire" costs the equivalent of that person's salary. We've seen examples where the damage done by a "bad hire" far exceeded the cost of their annual salary. Our experience is that the best time to do an in-depth investigation into a prospective employee is right after a conditional offer of employment has been extended. If the person has survived your pre-hire background check, the post-offer background check (with their consent, of course) is where the expense of a thorough background check becomes worthwhile.