I wasn't very high on Halladay going into the season (due to the control problems that had plagued him in the past), but I'll admit I was wrong. The only question now is whether he'll tire later on in the season, since he's never had to pitch more than 150 major league innings in the past. This makes him a slight risk, but I think he'll be fine. Halladay's wins might suffer a bit in the second half, however, if the Blue Jays do decide to drop significant payroll. I'd say he's worth Spivey, but you should at least see if there are any higher offers for one of your closers.
I'd rank the others you list Hudson, Morris, Astacio, Colon. If any of your closers can get you either Hudson or Morris, I'd strongly consider making such a deal.