I hope valve reverses this somehow, because this is a godawful idea.
There's paying for the game.
Then there's paying for the DLC (which some of the mods listed as costing money require)
And now there's paying for mods.
Some people get games just for the mods and the replayability/game time they add. Now people have to shell out money to get the one thing they wanted. For pete's sake there's a mod that adds a single article of clothing that costs 75 cents. This is no doubt going to spread to other games and just might kill modding as we know it.