But then there's the opportunity cost that comes with using cap space as opposed to a S&T. I know you understand better than most of us the benefits of trading for Kirilenko as opposed to signing him outright. I doubt the Spurs really cared about the actual salary they were paying Kirilenko. They probably weren't willing to give up so much flexibility for him, though. By using cap space, the Spurs would not have been able to get Pendergraph, and they would have had to give up Neal's rights. They would also had to get Ginobili to take a smaller deal.
By trading for him, the Spurs would have left all of those options open while also being able to give Kirilenko a bigger deal for the same player movement. Amnestying Bonner and trading De Colo for nothing would have given the Spurs $5.4 Million in cap space, while also killing their MLE. On the other hand, trading De Colo and Bonner for Kirlenko, would have given the Spurs $8.1 Million to offer.
To me, this signals that
Mel_13 is correct that the Spurs just didn't want to pay that much for Kirilenko. As silly as it sounds, they may not have thought he was worth losing Neal's rights and potentially alienating Ginobili, not the mention the picks it would have taken to make it all work.