Currently Horizons isn't worth the price unless you really want to drive around on airless. If that was you, I'd assume you'd have bought it already no matter the price, so my advice would be to wait at least until 2.1. Maybe the 2.1 content and mechanics - and more importantly how it's all implemented and balanced - will make it worth for the less explory types, but that's still speculation.
Regarding sales, as others said, it will go on sale at some point. 2.1 seems to be quite chunky, so Frontier will probably advertise it a fair bit. And considering the steam summer sale will come at around the same time as 2.1, it's quite likely Horizons will be part of it. Nothing drastic like 50% off, but I'd be surprised if it doesn't go on sale at 15% off.