For the Rangers, they would have to pay less money on the open market for Yu as the #1 team than they would under the posting system.
that would be the opportunity cost- right now, it costs them the posting fee for the extra year and the exclusive rights- it might not be cheaper next year because he could decide he wants to be a yankee/red sox, live in a certain city, pitch in a friendlier park, play in the NL... or any number of other things that could have him playing elsewhere.