TL;DR - I'm debating whether I should invest $13k in a moisture barrier in my crawl space.

I recently purchased a home in Santa Cruz, CA. The neighborhood is known for having a high water table, and the air has moisture from the ocean. Our home inspector noted that there was mold on the floorboards in the crawlspace. I went down there to take a look, and indeed there is some white powdery stuff on the floorboards. We got the inside of the house tested for mold though and it came back clean. I started looking into moisture barriers and had a couple companies come out to provide estimates. Water marks on the foundation suggest that there have been inches of standing water under the house at some point. They are therefore suggesting a 20 mil moisture barrier (on our 1750 sq. ft. of crawl space) plus two sumps pumps with trenching. The quotes are for about $13k (excluding electrical work). Yikes!

I got back in touch with the mold inspector who said that he does NOT recommend moisture barriers because (a) if the house was built in the 1980s and mold hasn't made its way from the sub area into the house, it's probably fine and (b) mold would certainly grow on the underside of moisture barrier so if it ever ripped we'd be in a worse situation than now where at least we have some ventilation going over the ground down there. Of course the moisture barrier companies claim that the moisture barrier is absolutely necessary for protecting me and my family from more extensive mold growth.

Does anyone here have an informed opinion on the matter to help us decide whether it's worth it to make such a large investment? For what it's worth, we plan to be in this house for a long time (e.g. 20 years, though you never know what could happen). Thanks very much!