You do *not* have an average DPS of 200. You have a *burst* DPS of 200. The item runs on a 30 second cooldown. The average DPS of the active ability is about 9.5. It doesn't seem that great. A burst DPS of 200 isn't that great anyway.
Look at it this way then: the -400 mana effectively negates some 12 mana/sec, which - if you ask me - is quite a lot. Then again, burst damage is often more useful than DPS, as DPS the enemy can calculate with and heal/regenerate through, while burst damage synchronized properly with other player's abilities can punch trhough any healing he/she has.
As for boxleitner's comment:
If the game is 3v3 or bigger you will most probably be swimming in money by the time you get to giants. In 2v2 you really have to put every gold aside for cit upgrades and the bare essentials in items. Still you have to consider this one too: i might be sacrificing 1000 or even 1500 gold when using an item i will later sell, but if that item is the right one, it can easily lead to me getting a kill on someone which instantly pays for any costs i had in the process... Not to mention, that thanks to my superior gear im able to hold the lane forcing the opponent to retreat, resulting in me and my team getting to warscore 8-10 or even just 3-5 well before the enemy.
Its not just the creep upgrades that do matter.. Getting those gold upgrades earlier than the opponent can also have its effect on the game.
When the time comes that i need upgrades, i can just sell items until i have enough money for whatever upgrade i need. At that point, the focus of the game shifts anyway, with the goal being to secure as many portals on the map as possible, so individual demigod strength does not matter that much anymore.. its more like survivability and mobility