I understand and I have seen this in other games like LOTRO (buy 3k TPs, get 1k free). Still, gems you buy in BWE3 will be carried over to the release and it would be nice to see any bonuses you might receive during the actual purchase.
The price per CS-currency has always, in every cash shop I have seen, been set firmly. You can probably confirm this easily on a CC statement where the CS-currency amount has been noted. What people think of as discounts are attached bonuses, but they are "separate" from the payment, if that makes sense to you (i.e. you pay for X(m) gems and receive a Y(n) bonus where Y(n) depends on m). This would be my reason as to why we are not seeing any discounts - the bonuses are not visible yet. As an example, one pays Z for 4000 gems, as per the base rate, and then receives Y(n) as a bonus, difference being that the bonuses are not visible here (yet?).