Crawl space dampness

Property slopes down from east to west with the east having issues with musty smell. modular home on foundation and past leaks in both baths that are at the east and midway through home. decks at east end of house front and back and noticed an unusual amount of flies this spring/summer, assuming due to dampness. I have had several estimates and everyone wants to do something different and ranges from $2300 to $9000. Is there a basic rule for this and why are estimates not broken down into supplies and labor?