As I write this, Dungeon Defenders costs $14.99 on Steam and has $39.84 worth of DLC. This soundly trumps last year’s post on Civ V, and it is something I did not see coming for this game. Only a few dollars of that is purely cosmetic; it includes new classes, missions, holiday events, and conventional expansions. At the end of last year, you could get the whole game with all expansions for $6.20 on sale. If this works for them, Trendy Entertainment has found a highly profitable variation on the cash shop model.
Of course, this comes nowhere close to Railworks 3: Train Simulator 2012. As I write this, it costs $34.99 and has $1,869.03 (not a typo) worth of DLC. It seems to be priced to be competitive with buying actual miniature train sets.
Update: as commenter Galaji mentions, the Dungeon Defenders holiday event DLCs are free if you get them at the time. I was rather surprised by the Presidents Day event.