It'd be more useful to track five of the top selling artists over the past three or four years and look at the monthly average sales price...
(1) Each sketch is unique - some just look better than others
(2) Auction titles aren't standardized - The artist name/set title/character name isn't always present
(3) Some characters sell better than others - Spider-man sells for more than Squirrel Girl
Collecting Cleveland Browns ( Josh Cribbs, Kevin Johnson, HOFers, Contenders, National Treasures )
Want - http://www.blowoutcards.com/forums/football-singles-buy-sell-trade/442587-buying-browns.html
My PC - http://s39.photobucket.com/albums/e177/skaughtto/Browns%20Cards/