I would bet that they do sell at a loss. There's an interview with Gabe[1] in which he keeps repeating that the price point was painful. He is very vague about it but he also mentions some calculus like "they'd have to sell 8 games to be profitable". So I have no doubt they're doing it like the consoles do it.

[1]: https://www.youtube.com/watch?v=4FXgDAF6QpM