Yup, Horizons should be included in the new Era, but it should cost less for people who already own it.I'm hoping they've only got two items on sale;-
1. Full package - which is Elite, Horizons, Beyond and new era.
2. Upgrade package - which will upgrade you to New era, including Horisons if you've only got the base game.
Horisons will have been out almost 5 years, it seems silly to split the player base any further. Maybe charge a little more (seriously not much) if people need Horisons as well ( just if you're feeling salty about it) but make sure that everyone who upgrades or buys new is at the same level.