Lets face it - this should be more of a "Monthly DM subscription costs to play D&D", since there's at least one book a month being released. I'm just giving the rough month to month - for more exact dates and costs, check out Amazon.com

May - Monster Manual 2
June - Eberron Players Guide
July - Eberron Campaign Guide, Divine Power
August - Adventurers Vault 2
September - Dungeon Masters Guide 2
October - Primal Power
November - Draconomicon 2
December - The Plane Below

Amazon's prices seem to vary between $19.77 to $26.99. So it's better then cover price, but if you get the free shipping, you'll be waiting 5-9 days for your book to get there.  But it might be something you can barter with your local game store with to get a pre-order price reduction from them.