The shiny stuff blocks more sun than without it. Thin fabric tends to let in UV rays, even it is waterproof. By metalizing the fabric, it can effectively block all the UV rays. It costs more to metalize the fabric as well, which makes the umbrella more expensive. With the amount of surface that is metalized, 30 bones ain't bad.
_________________________
I've taken a vow of poverty. To annoy me, send money.