When people say they have dry live rock, that is now "base rock" Meaning it was live rock that was left out of the water and does not contain beneficial bacteria and critters. In theory it should be cheaper than live rock.
If you look for live rock from MR reefers, you can probably get live rock from a low of $2/lb which is hard to find, to a very easy find at $3/lb. LFS sell live rock for a higher price.
Some people think that buying base rock at a reduced cost is not worth it, seeing how you can get live rock for $3/lb.
You are correct that in an established tank with a good amount of live rock, you don't need to purchase additional live rock and can use base rock that you have cured. You can cure base rock and place a piece of live rock in curing bucket and that can help seed rock before being placed in display tank.
You don't have to seed base rock, however, you should always cure any rock, before placing in your display tank. So yes, you can buy base rock at $2/lb on ebay, or just go to the For sale section of MR and buy from a local reefer.
Now, just to add a kicker. I have read that it is beneficial to get live rock from multiple sources so that diversity will occur. So that it would help your tank, to get some more live rock, since it will be coming from a different source and that you should switch live rock from another source every once in a while. This is supposedly better for an established tank.