There was a lively debate on this with some Piston fans on another thread earlier in the week. They greatly undervalued Rondo, but were clear that Drummond was the guy they wanted to keep. They wanted no part of Wallace's terrible contract. It was clear the fan base wanted to just dump the expiring contracts and give us Brandon Knight for Rondo. Celtic fans were equally adamant that there is no deal for Rondo without Monroe or Drummond on the table with Knight. My guess is to get one of their bigs and Knight, we have to offer Rondo plus draft picks.
For what it is worth, Knight, Drummond, Monroe and Caldwell-Pope are all lottery picks. It takes a long time to build a really good team. Despite that talent, Calderon bolted for Dallas. They got Josh Smith because he had nowhere else to go once Atlanta signed Milsap. Atlanta was in a better financial place to just let him go. Most of his suitors got their guy, so the Pistons were left.