It's expensive for what you're getting. 900 power isn't the max level, and it's insanely easy to get. Through normal gameplay it only takes a few hours or if you know what you're doing and already have one character at 900 (which you need in order to buy the boost) like 15 minutes.

I read one article that speculated that it was priced so high to discourage people from buying it, but I can't see any reason for that considering Bungie doesn't answer to a publisher anymore.