I don't think it's overly unreasonable. Neither do I think Blizzard is getting overly greedy. Let me explain.
For comparison:
Call of Duty Black Ops 1. On Steam, this costs USD60. Each of its DLCs costs USD15. What does each DLC give you? 4 multiplayer maps. CoDBO1 released 4 DLCs for a total cost of USD 60.
Call of Duty Black Ops 2 also costs USD 60. Activision (Blizzard's parent company) says they will also release 4 DLCs priced at USD 15 each. But they offer a "season pass" where you pay "only" USD 50 to get all 4 DLCs when they come out.
EA Games' Battlefield 3 costs USD 60, each of its 5 DLCs cost USD 15. Sims3 costs USD 40, and its 11 expansions cost a total of USD 320.
Ubisoft's Rocksmith costs USD 50. All of its DLCs cost a total of USD 370.
CoDBO1 cost a total of USD120. Then 2 years later, they release CoDBO2 so you could spend USD120 again. You just spent USD240 on 2 games and a bunch of maps that 2 or 3 years down the line you won't play anymore, because a sequel will come out. I could say the same thing about BF3.
Circa 2016. SC2 Legacy of the Void expansion comes out. I will have already spent USD 180 for the whole SC2 series. But this is one of those games that I know I will be playing until SC3 comes out in.. well, 2030 probably. Heck, I still play SCBW with my brothers and cousins!
Blizzard has really built up on the success of SCBW and they know that nothing can replace StarCraft as THE RTS game for the competitive scene. Meanwhile, I don't see (or at least, know of) a competitive scene for CoDBO2 and BF3.
So yeah. This game will cost me USD180. But I know I will play it for a loooong time, and I know Blizzard will continue to support it because it has an active professional scene and a very large community following.
Blizzard just has to step up in regard to game balance. But that's a different topic altogether.
Just my two cents.