Because Castillo's contract is still the Mets' responsibility, the Phillies will only be on the hook for the league minimum of just over $400,000. And why would Philadelphia make this move? Because Chase Utley is limited by a knee injury, and with the star second baseman making very slow progress, the team needs a stopgap. Wilson Valdez would be one possibility, but he's best in a utility role, and the front office doesn't have the budget flexibility to add anyone of real impact (like, say, Michael Young).
So, assuming the report is true, we'll see how this goes. The Phillies will be free to release Castillo at any time at minimal cost, so this isn't much of a gamble. And Castillo can still draw a walk and put the bat on the ball as well as anyone. While he can't run like he used to, Castillo's OBP over the last three seasons is .366, so he might be able to help the Phillies for however long he's around.
From one team in the NL East to a better one, apparently. For more on Castillo and the Phillies, check out The Good Phight.