So it looks like Discount Comic Book Service has the lowest price on Absolute Batman: Hush ($27.49, even though I've seen DCBS advertisements on the web that say it should be $25.99), as well as the lowest price on a couple other trades. As far as Hush is concerned, Dreamland Comics comes in second, with $29.99, and if you spend one more penny, then you don't have to pay shipping as with DCBS.
At the same time, MailOrder Comics seems to have some pretty low trade prices (Absolute Batman: Hush aside), but they've just re-done their site and for the life of me, I can't find their shipping prices, so it's kind of hard to tell.
Meanwhile, Dreamland has posted Prelude to Infinite Crisis for $3.59, while DCBS has it for $3.89. No sign of it at MailOrder Comics.
And while we're here, I'd like to give a shout-out to Grasshopper Comics. Their prices aren't very competetive, but they do post photos and transcripts of both the front and backs of their trade paperbacks, which is pretty cool if you want to browse or check out a new release. So thanks!
Found a deal on trade paperbacks on the 'Net? Let me know!