I think some ppl kind of thought this was SD/IC trying to take money off fans for a glorified patch. But seeing as a full expansion pack usually costs around $30 anyway, you're getting effectively more than a third of an expansion (in terms of the stuff involved) for a third of the price of a full expansion.
I mean, would you rather have the entrenchment stuff now or when they've finished the others?
Any comparison to what Blizzard is doing with SCII is interesting. I'm going to reserve judgement for when the second part comes out - I would be surprised if they could get away with any price more than that for an expansion.