Hi,
what happens when a player plays the negotiation card multiple times in the same turn? Is the next skill card 2¥ less per skill card played that turn?
I think PiXeL01 is right. If you're talking about the card main ability, the rulebook (p. 23) says that " This card counts itself toward the discount." Logically then, playing two Negotiation cards would mean that you have a discount of 2 indeed (or 3 or 4, depending of the total of Skill cards you played this turn) if you buy a Skill card this turn.
If you're talking about the card Assist ability (one of the best Assist, imho), playing 2 Negotiation would mean also (imo) a discount of 2, but for any card you buy this turn.