Why don't they do the background check before the interviews so they don't waste eachother's time and money just to later have a background check fail. Does a background check costs more than an interview and paying for the flights?
Not a for a single candidate, if you however need to do it for all candidates before they are evaluated it will be much costlier :). Doing it after the interview for few candidates is much cheaper.
Also it may take weeks to do the full check, you will loose the candidate if you wait for so long to do the assessment .