If you purchase this product under the Scott's Miracle-Gro label in it's zipper-close plastic bag, it'll set you back about $12 per cubic foot. If you purchase this product at a builder's supply aimed at the concrete block/masonry industry, you can probably get four times the quantity for three bucks less. Ideal if you need to fill a large raised bed for veggies or herbs.
Of course, if you were to add this product to your soil, you'd want to know what it does. First, it's nearly PH neutral. Second, it encourages drainage to such an extent that it is used in many professional mixes (sometimes in excess of 60%) for rooting cuttings or starting seed. Not only does the product encourage drainage, but it allows air to reach the roots (necessary!). Recently, this product has seen a resurgence of interest because of its usefulness to green roof technology. This product is considered, in soil, to be an "inorganic structural element"which adds value to the organic elements (like compost) by creating those air and water avenues that allow nutrients to reach the plant roots. It is inert, sterile and lasts for years. It is made into its commercial form by heating siliceous volcanic rock, which causes the water in the rock to expand the particles into lightweight, insulating Power Dirt.
OK, I made that last term up. Seriously, the sources like to say that it expands to about 20 times its original size and pops like popcorn. Somehow, when I opened the bag, it did not make me yearn for butter and salt.
So how much to use? Though the site design is a bit archaic, I'd check the perlite.org link and the Soils for Containers and Bonsai link for some specifics on the best course for your application. Based on my reading, I'd say you need to start with an idea of your own soil requirements. If you have heavy clay, lay down a layer about 4" thick and either till or fork it into your soil for general vegetable gardening. If your turf needs work, they recommend having the yard spiked/aerated and then raking the perlite into the newly established holes in the yard. If you are building raised beds specifically for plants like herbs that need really well-drained soil--well, some folk are using containers of 100% perlite. I don't support that idea because then you have no choice but to fertilize and water constantly (fine for commercial drip applications, but a bummer in the yard!).
Potential uses? Rooting. Seed starting. Green Roofs. Stormwater filtration. Container gardening. Making golf courses more drought resistant. Improving clay soils for agriculture...or home gardens. Filtering pond or aquarium water. Growing orchids. Or bonsai. Perlite can be your secret weapon. Why, it's almost as miraculous as compost! Which, I hear, can benefit from the addition of perlite! Especially if you don't like having to turn your compost. ;)
Oh. And it insulates concrete block walls and makes them more fire resistant. So go find a concrete block dealer and buy the big bag!
Resources: Perlite.org, Vermiculate vs. Perlite, Soils for Containers and Bonsai