I ordered my last round of rock from marcorocks.com. I heard for a while that they had some supply chain issues, which I have also heard have been handled.
It comes dry and is incredibly porous. Because you aren't paying for water weight and there's so many holes in it, 50 lbs was more than enough for a 90 gallons. With tax and shipping I got the rock at my door for less than $3/lb (around 2 years ago, not sure how much it's changed).
On the issue of biodiversity, I specifically bought dry rock to avoid a number of specific critters and algaes that can come in on live rock.
I don't intend to start an argument by saying this, but I think biodiversity is a little bit overblown when talking about our tanks.
A relatively small amount of the stuff that comes in on the live rock is actually going to beneficial and thrive in a captive system. A lot of the stuff will die off in the first couple of weeks from transport (e.g., the vast majority of sponges) and quite a few others will die in the weeks afterward because they are ill-suited to the tank conditions you are trying to establish. Finally, specific species are going to be better suited to the tank conditions an inevitably one or two will come to "rule the roost." Some hitchhikers are more than able to reach plague proportions in a tank (specifically algaes which are happily fed off of the die off that will usually happen with uncured fresh rock). Also, for every one or two species that are beneficial, you're probably bringing in 1 or 2 that aren't. Getting a rogue crab or mantis shrimp out of a large tank can be nearly impossible.
Also, as you add corals to your tank, you're likely bringing in small amounts of substrate from other tanks with them. This is a blessing and a curse from the standpoint of hitchhikers. It's a good way to increase the diversity of species in your tank. If those species just so happen to be coral pests or otherwise undesirable critters, you've just added that to your tank. This is, of course, why it is recommended to QT and this is where you can let the good stuff through and do your best to prevent the bad stuff getting in. It's a lot easier to do that when you're dealing with small bits of rock/corals than it is to monitor 100 lbs of rock.
Of course, it all comes down to what you can afford, what you're willing to deal with, and your own philosophy on what you want your reef to look like. I prefer to start with a clean slate and try my best to control what goes in my tank as opposed to starting with a whole bunch of stuff and having to weed certain things out.
