Republican governors of the teabagger variety are falling all over themselves to prove their extreme bona fides by vowing to forego lots and lots of federal money that will provide increased Medicaid coverage for their constituents, not to mention vowing to let all those people who won't be getting the coverage to just fend for themselves. These governors say that the expansion will lead to a major budget hit, eventually, when the federal government could stop picking up the full bill for the expansion.
But, via Greg Sargent, here's a 2011 study [pdf] from the Urban Institute that shows just how much states could save in other budget areas by taking on this expansion. The bottom line: "state would spend $92 to $129 billion less under the ACA than without it [...] between 2014 and 2019."