If I am not mistaken, and if you bought the 2015 LEP, you would also have to deduct at least the price of the vanilla game (unless you had it already in which case you got a reduced LEP price), plus the Horizons standalone price and the price of the beta access upgrade, all at the time you bought the LEP, for example.
The 2015 LEP was offered for sale at the same time as the Horizons pre-order, both included the base game, and if you already owned the base game you got £10 (GBP) off. I paid £120 for the LEP because I already owned the base game, so the base game value is not a part of my LEP. Horizons is, the Horizons beta access is, that's it.
I think it would be great for someone from FDev to confirm this stuff, it isn't a secret.