So it's a vented crawlspace. A traditional bad idea. (See Building Science Corp. above.) But then you're using the crawlspace as the supply for the HVAC--in other words, forever sucking in humid air, but at the same time trying to dehumidify it. Sisyphus had it easier! Your crawlspace should be conditioned. In other words, it should be treated as part of the living space. You already are using it as a source of supply air for your HVAC. That means no vents, non-fibrous insulation at the perimeter (none on the crawl ceiling), a high-quality ground cover (not just sheets of poly) and provision for dehumidification, at least for the first few months after occupancy. Here is a link that might be useful: Green Building Advisor on Unvented Crawl Spaces

eibren, as often happens when industry designs change, so do commercial and residential spaces typically not called upon in the past such as crawl spaces. I have a mid century Colonial close-in to a major city in the Mid Atlantic with humid/wet summers and recently increasingly wet/cold winters. Our crawl spaces were added in mid 1970s. GreenBuilding design is catching on even in crawl spaces, widening the debate to vented vs unvented and if the later, conditioned space. I'm changing my crawl spaces to unvented, conditioned space after extensive reading and thinking on our crawl spaces. I'm fixing problems which need fixing and the process requires light. Surprisingly or not, all who have given quotes have said lit crawl spaces are increasingly common to better keep an eye on them and to make them more inviting/inclusive. I've spent many an hour dragging LED portable battery rechargable lights (great light btw) around while working, so it will be nice to flip a switch or two. That's my take on it anyway.

Once you landscape, you should solve most of your water problem. Adding exterior water proofing and drain tile around the entire foundation to exit on the down hill side will solve the rest of the intrusion into the crawl space.There really are no shortcuts in dealing with active water intrusion into a crawl/basement space. Preventing the water from entering is really the only sure way to maintain a dry space. As for the radon abatement, there are lots of solutions. Passive abatement is the least expensive long term. That means installing a grid of drain tiles(holes downward) and a stand pipe(extending to a height above the roof) under gravel. Then the plastic you mentioned and pour the concrete floor. The passive system is done without the concrete, with the plastic sheeting glued to the foundation walls and any pipes. Pouring a floor over it is fine., but the sealing is still very important, as concrete alone will not effectively seal against the gas. Which is forced from the earth naturally by pressure in the earth. Even though radon is heavier than air, the passive system does not allow the gas anywhere to go save into the piping and eventually up the stand pipe and out into the outside.

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.