It wouldn't necessarily be about the money. This would provide both teams with better geographic rivals. Milwaukee v. Minnesota/Detroit. Kansas City v. St. Louis. This would lead to more road fans travelling - good for baseball and, as Dry pointed out, the gate.
Would also cut down on the travel time for two geographic outliers in each division. I think it would be a good move for both clubs.
It could happen as soon as 2009 as well, so talk of current competition is important.