I would go as far as to say, haunting guise is on the same level as Nedlessly large rod and BF sword.
You pay big sums of money for big stats, except in this case you can actually buy parts of the items first.
And mages have other cheap options if they need certain stats.
If they need HP they can get ruby crystal, if they need ap they can get tome/blasting wand (both of which build into liandaries) They can get 15 magic pen + move speed for 1100 gold.
So yes, haunting guise is expensive for a tier 2 item BUT its pointless to reduce its cost because of its decent build path and many other options ap champions have.
More over many champions derive their power spike from completing haunting guise, if haunting guise were to reduce in cost and loose stats many of theee champions would be shafted and it would create a fresh set of balance issues.
TLDR :
Haunting guise is fine as it is, its not a balance issue and there is no reason to change it.