With that strategy alone, they're likely to surpass a $20 million prize pool this year, which means $60m in pure profit for Valve.
The impressive part is that none of the compendium affects gameplay whatsoever. It's solely things like cosmetics that make your character look cool.
It's tempting to call Dota a one-off, but I think this model could be followed by many other games: make your game competitive, then fund a yearly tournament. The reason Dota is so huge today is because Valve personally seeded the first International's prize pool with a million dollars, which was simply unheard of at the time.