Good question this. I think the 2009 season had a greater focus on the Johnson/Rioli prize wildcards for that season. If you finished in the prizes, there was a greater chance you would get those two prize wildcards than a team set (They even came in their own little case and certificate). This would be the same case for 2010 and 2011, although as we know, 2011 had a prize leakage problem, and 2010... If I remember correctly, I believe they expanded the winning places to twice the number, which would carry on to 2011 and beyond.
Good thing I got myself a full 2009 prize set, which includes the last premiership prize set in Hawthorn, which is one of the most sought after prize sets.
No matter, I'd say any prize set from 2007 onwards is plentiful, with 2009 being the major exception to the rule.