RoS had 3 major selling points - a new act, a new class and new features (i.e. Mystic, Adventure Mode etc.). Personally, I'd say that a whole act should be worth more than a class and we should also count the promise of new patch features to be added later on as another selling point, but... hell, let's be generous and assume that there are only these 3 (class, act, features) and that they are all worth the same.
Based on that, since Necro pack is basically mostly a new class, I'd say that a fair price would be around 1/3 of the standard expansion price. RoS cost 40 USD at launch, so the Necromancer pack should cost slightly less that 15$.
That said, I believe that 14,99$ is a fair price, though personally I think Blizzard will try to milk people and go for 19,99$.
As for me - if it's 14,99$ or lower, I'll buy it. If not, I'll just wait for the whole pack to go on sale as I really don't believe that it's worth more.