This isn't going to shock anybody, but the Astros' final payroll was pretty low in 2012. How low? The average salary in Major League Baseball was $3.2m. The Astros' average salary? $685,000. The Yankees' average salary was $6.9m.
Just to put this in perspective, MLB's minimum salary in 2012 was $480,000. Meaning that the average salary for the Astros was $205,000 above league minimum. The linked AP report says that the Astros' average salary was the lowest across the league since the 2006 Marlins.
As an aside: Of course it was. No team in full-on rebuilding mode should have a high payroll. In this case, the Astros got what they paid for.