There's plenty of land, the problem is that everybody wants to be in the same dense urban cores. Drive 3-4 hours north or south of San Francisco and you have lots of cheap, beautiful coastal land available.
Inspired by this comment I actually spent 30min looking on Redfin. Most of the coastal land I found is listed for around $1-2M (for ~1 acre), from SF area down to LA. Check it out! I was actually pretty surprised by this.
I really can't tell whether you find that to be surprisingly cheap or surprisingly expensive.
Where, specifically, 3-4 hours south is the coastal land cheap?