Graxy- I'd agree with those points costs as being about fair, but I think the hellflayer is the best choice not the worst. Remember you are getting an exalted alluress as well as the soulscent rule.
Hellflayer has scythes so its 4.5 impact hits on average. If you pick your target right thats 3-4 extra attacks - combined with the 4 extra attacks for the exalted I'd say 20 points extra is fair enough. Your only real competition in rares is the soulgrinder, Beasts or fiends (juggers are not good for the points - die far too easily).
Edit - Gandalf is correct - for your limited edition set you actually get less value for money. I remember when Empire had a wave of releases a few years back they issued a limited set that actually did save you money. Also the Eldar WarWalker had a deal where you could buy 3 in one box and save money. Not quite sure why suddenly buying 2 should cost the same with 1 less base? £30 would have been a more appropriate price point.