It's very, very hard to try to put a percentage on something like this, so take this with a grain of salt... All things considered, I'd say the odds of a trade involving Alfonseca are slightly higher than 50% (55-60?). Assuming Gordon's recovery goes well (not quite a given yet), shopping Alfonseca would make sense for the Cubs, both in terms of their needs and their salary situation.
The recent rumor involving the Angels may or may not actually come to pass, but there's enough logic (as well as confirmation that the Angels are indeed interested) to take it seriously.
Keep in mind that even if he stays in Chicago, he may wind up playing second fiddle to Gordon. On the other hand, it's also conceivable that he is traded to a team that hands hiom the closer's job. There a lots of unknowns in this equation, not the least of which is the status of the talks between players and owners...
It's a tough call now that you have Castillo. If butler keeps playing the way he has so far, at least at Coors, he'll be worth a closer with greater job security. Of course he might also fizzle, leaving you with nothing. Either way, it's a gamble.
If you can, see if there are any better offers out there for Butler. Barring that, try to acquire Tom Gordon. If you succeed and complete this deal, you'd be guaranteed the Cubs' saves.
If neither works, you're left with a tough choice. I'd probably stick with Butler - that's a great price for a player of his potential in his situation, and the odds that he'll lose his job seem smaller than those of Alfonseca losing his.