Valve takes a % (about a third), but the thing that really hurts small-amount sales is the fee charged by whatever payment method you're using (usually a credit card or PayPal). On a $1 payment, the fee is usually something like $0.33. So if you buy a dozen $1 games, the amount is aggregated and the payment fee is negligible, but if you just buy a single $1 game, the developer gets something like $0.15 (assuming they have a publisher with which they have to split sales, as Firaxis does with 2K). Also note that if a game is returned, it's the developer that has to eat the cost of the transaction fees.
This is why microtransactions don't work very well unless they are aggregated, and why the proposed Patreon changes were such a big deal.