The term base rock can also refer to rock that was once live rock but has been dried out. It is generally much cheaper and can be "seeded" by putting it in a tank with established live rock. Over a period of time bacteria, coralline, microfauna, etc will grow on it and it will once again become "live" rock.

My 30 gallon tank that I set up in July has around 40 lbs of rock in it, 34 was this type of dry rock and the other 6 were live rock that I picked up from Aquarium Designs.

marcorocks.com and bulkreefsupply.com are two online vendors that sell this type of base rock. The base rock that I am using came from a vendor that is no longer in business - hirocks.com (hawaii rocks), I had some extra in storage.