We all know here the usual "list price" from Boeing and Airbus are during a sale discounted by 50% or more.
Do you guys know if the usual 50% discount apply aswell with ATR / Embraer and the others ?
Your assumption is false and has no basis in fact.
Boeing has a forumula from which to work with and its a closely held proprietary one at that. I had it explained once, but I do not recall the exact details, but it sort of follows this theme
If you are an existing operator of Boeing aircraft and you are adding the existing fleet you are at the top tier of customer discounts.
If you are an existing operator of Boeing aircraft purchased on the open market (used) you fall a little further from the top.
If you have never operated a Boeing aircraft, but are purchasing new this time, you may be a notch or two lower.
If you are buying a used Boeing aircraft you are further down the list.
There are about six iterations of this theme and each one has various benefits. Obviously an operator like SWA is in the top tier category. Someone like B6 may not have the same bennies unless there is a "sales campaign" in effect that drive a no less favorable bargain. Again this is not a blue print for Boeings sales force, but rather a conceptual footprint.