I'm in Dallas and got a quote to do ours for $12k. Our house is about 3500 sq ft. We ended up doing it ourselves for $2400. That included 20 mil plastic for the ground, walls and piers and a dehumidifier.

I just put Stego wrap under our poured concrete floor and it's often used for encapsulation of crawl spaces. I am really pleased with the product and it's 15 mils, and a radon barrier (not a retarder). Used the Stego tape as well - and they have a VERY sticky version for crawl spaces / sticking to rough surfaces - highly recommended. Handy folks could DIY this easily as gthigpen notes.

That price seems crazy - but... the again - it's Cali. And if your crawl is low, it's a huge PITA to work in there.

FYI, We just bought a house with the complete encapsulation package. While we live in Georgia which has a much higher ambient humidity than many areas in California, be aware that the humidity in the space will be high. You probably will need to install a humidifier system on top of the basic encapsulation. We had to do it as our humidity was above 70%.

Our system included a spray on foam as well as the plastic sheeting. Understand that people answering your question may have a different system with different materials. Thus, the differences in cost.

Ours is $3500 with a first floor about 2000 sqft. This is in NC. So we insulate with foam. Not sure if that is an all inclusive cost to be honest but it is most of it.

So IMO, the bay area should be roughly 200% of our cost so I think the price is exorbitant and unreasonable.

National chains for things like this are not a great idea. I'm not sure why you would even need to seal in No Ca. What is your motivation? We like humidity temperature control - neither of which you need. We get lows of 20 degrees in the winter.