It also depends if you want the single set packaging with COA or the lens only. The lens only is about $2-$3 cheaper than with the box. 2011 is running about $15 and the 2012 is about $50-$55 per the reasons denco7 mentioned.
This set is almost always cheaper on the aftermarket (except for 2012) so it never really pays to buy it individually from the Mint for $18.95. Many collectors and dealers break up the standard proof set for the individual coins, so the single lens is almost always cheaper than the offering from the Mint.