A better question might be who could the Sox have spent the same money on (say 14m) who would provide the same value in wins. I haven't actually searched but someone could research using WAR. There is no doubt his cost is high, but many baseball economists (and teams) agree that paying more for a player whose wins put the team into the playoffs (and the extra revenue and acclaim post-season games provide) is a reasonable strategy. If the Sox believe--as I do--that this is a playoff team, and that replacing ortizs production would cost more than 14m or decrease team flexibility moving forward, then perhaps it makes sense. Remember, the comparable signings would have been multi-year signings and would have actually cost more.
The move makes no sense if you think the team as constructed in 2011 was incapable of winning a WS in 2012 with some minor tweaks. I suspect the Sox FO thinks the team they have is very good, and that losing Ortiz would have created more trouble for 2012 and beyond than it was worth.
They had limited spots to fill on the team, they have a DH in waiting for 2013, and due to having limited spots open they had limited options in terms of who would replace that production. Could they have purchased Ortizs production with a combination of RF and SPs as the market has played out thus far? I don't know, perhaps. It may have been risky though to put their money on Beltrans market not becoming excessive, or Buehrle agreeing to a contract that comes close to reflecting his actual abilities, not ace status salary.
Overall this is an interesting discussion. I bet with events happening just right a better combination of players could have been found, but I don't think they could have doubled their value for the same salary, and this way if they really DO think this is a club in win-now mode, they still have a beast in the middle of their lineup who is only signed for one season. Higher coat, lower risk, 2012 still a very good team.