Because California is so restrictive in what it allows companies to do and not do, there's no way I could even ballpark a price for that work there. You'll need to talk to a Cali company to get a number that's close.
For the sake of arguement, I'm going to assume "conventional block" means concrete masonry units - the ones that are 8" tall, 16" wide and 8" deep with 2 or 3 large voids running vertically through them. Like this:
http://www.ccmaonline.com/concrete-h...ete-blocks.png
Gravity walls will be pretty much any retaining wall unit made by Keystone, Unilock, Versa-Lok, Anchor, ICD. I believe Keystone has a pretty big presence in your area.
For cost, the CMUs are going to be much cheaper to buy, but far more expensive to lay/build. Frost-free footings, masonry work are going to make the project take more time and more money than a gravity wall. The gravity wall blocks will cost more initially, but once the first course is up, those walls build much faster. You'll likely need geogrid, and it wouldn't be a bad idea to talk to an engineering firm about the wall (or at least an engineer at the mfg of the block you select).
Hope that helps.
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