Cost Analysis = cost 850 gold
Gold Value
20 ability power = Gold 435
10% cooldown reduction = Gold 266.67
50% base mana regeneration = Gold 250
Total Gold Value = Gold 951.67
Gold Efficiency*
Eye of Frost item Eye of Frost is 111.96% gold efficient without its passive and active effects.
active effect gold cost = sight stone value = 400 gold
gold efficiency with sightstone
total gold value 1351.76
=159.03% gold efficient

Cost Analysis*Edit
Gold Value
35 ability power = Gold 761.25
10% cooldown reduction = Gold 266.67
200 health = Gold 533.33
50% base mana regeneration = Gold 250
Total Gold Value = Gold 1811.25
Gold Efficiency*
Remnant of the Watchers item Remnant of the Watchers is 100.63% gold efficient without its passive and active.
with sightstone cost efficiency priced at 400 gold (becuase we already bought the ruby)
it is
122.8% gold efficent
and thats great and all
except i just payed 950 gold and lost 37% slot efficiency..
so why would i do that?
EDIT updated math... sight-stone passive value is 400 gold not 800 corrected math and numbers to reflect this