Based on item creations in Team Fortress 2 and DOTA 2
Valve has paid out $10,215,796 to content creators making items in Team Fortress 2 and DOTA 2. We knew this figure already, but what we just found out thanks to Steam Dev Days is that the payouts have gone out to 661 creators for a total amount of 2,349 items that have been created in this ecosystem.
As you can see in the chart, that's a huge increase from when they opened item crafting to users back in 2010. There are over 17 million Team Fortress 2 accounts now, with upwards to 500 million total items out there. Over 90% of the items in Team Fortress 2 are by the community, from models, maps, and more.
Oh, and hats. Lots and lots of hats.