People keep on saying it should been in two halves because Durarara is distinctly different from first half to second half. I never got that style of thinking, as it seems to me like the current DVD separation is a better fit. First DVD set is introduction episodes (first nine). Second and third, I won't say because the show hasn't concluded on Adult Swim, but they're different from each other.
Plus, I think Aniplex has money issues with Durarara, particularly concerning the dubbing. They usually go 50/50 when it comes to sub-only or dual track, but for Durarara, they hired some of the best voice actors around to make sure Durarara's dub is high quality (which it is), which means they're taking the series seriously. It's not like they're Funimation, who can treat FMA: Brotherhood (I own all Blu-Rays by the way) pretty cheap by comparison. That's my take on it at least, but considering that the DVDs are selling pretty well here, I doubt their regret their decision, and I doubt they'll change the price soon. And whether they'll release a complete set? Again, it'll have to be after this series concludes on Adult Swim for obvious reasons.
A story shouldn't just give people what they want, but also what they didn't know they wanted.