Few things before you spend all the money on this item.
Is the crawl space sealed or vented? If its vented and your suffering from excessive humidity you need to address that before you move ahead with the purchase. Otherwise it would be like trying to dry up the ocean with a sponge. Not going to work very well.
To determine what is going on I need some answers to some questions about the crawl space.
Is it accessible from inside or outside of the house, or perhaps both?
Is there heat or cooling system or ducts located in this area? What about plumbing pipes?
Is there insulation on the ceiling, walls or both? Fiberglass or foam boards?
What is on the floor? Dirt, plastic covering dirt? Cement?
Are there vents? How many and how large, Size of cement block, or larger?
How big is the crawl space? length, width and height, Approximate answer are fine.
Knowing the answers to this question will enable me to provide you with some low cost solutions before you begin to spend the money for the dehumidifier.
Check with the Basement Systems dealer in your area, they have units specially designed for your crawl space.