After this one ends January 3rd, it will probably be the summer sale, but Valve might surprise us. That and games randomally get put up for sale during weekends and the middle of the week, so who knows might go on sale again before that.
I take it you don't have a Paypal account? That's what I use to pay for stuff on Steam, since I don't want to bother with credit cards on there either.
That's (probably) the best price you're going to get it for unless you wait a year or two until it goes down. I'm probably going to grab it for myself.