Steam uses the steam wallet shit where you can only put certain amounts in it and you end up with like 50cents or 1dollar left. Luckily you can just sell some cards or whatever and buy some shit indie game for 3bucks next big sale to clean it out, mostly.
As for dota killing HL3, that's a load of bullshit imo. Steam killed HL3 long before dota was even a thing if we go the "money printing machine killed HL3" route. Why make games when you can sell games and make money. And then whatever ressources they had closer to HL2, they spent on doing TF2 and Portal. Great games and all but still probably took a bunch of time. Dota in comparison? All they had to do was the tech and HL3 was already a myth by that point. All the gameplay design and even character starting designs were already made for dota.
If HL3 wasn't released it isn't because all the artists were working on some dota cosmetics or whatever. It's because they could never decide on a good way to make the game, on a good story for it, the basic few first steps of development, which dota probably didn't affect at all since neither of these were really a large focus on dota(I guess there was some work on the dota2 lore, but shit's full of holes and disjointed things by nature so probably didn't detract much from it).
Especially in a company like Valve where people kinda decide on what they work and shit like that, if they actually had decided to go ahead with HL3, I'd assume a lot more people would have worked on that than dota, because having your name in the credits of one of the most anticipated video game ever(perhaps the most anticipated) would reflect a lot better on you. The fact is they never got past the early design steps because I assume they figured they might not be able to live up to the hype, and meanwhile they didn't need the money anyway. It's a risk with no benefit behind, you might tarnish your image and ruin a franchise(mind you, a franchise you're not using anymore, but still) and all that for what? 10% of the revenue of a Steam sale even if the game is a huge success?