Why would it be $30? I've grudgingly paid $60 for games that I was barely curious enough about, but this was one I'd have happily given $60 for.
Probably because Platinum games haven't done too great with Sega and they just want to get this one out and fulfill their obligations without pissing off fans... and possibly to get a lot more of them immediately. Honestly, the only Platinum game I'll get at full price will be Metal Gear Rising because the demo blew me away, I pre-ordered MadWorld at $40 (which is ahead of 90% or more of the people who bought it admittedly), got Bayonetta for $13, and Vanquish for $20. Will be getting this at launch simply because it IS $30, otherwise I'd wait on this too.