Hmm Id like to know the same. I just bought an Autozone one for $40. It works ok I noticed a little bit of moisture one morning on the rear and a little bit of dust, so it isnt 100% covered but its better than just sitting in my driveway.
OEM covers are not made for OUTSIDE weather! continuous use of the OEM cover outdoors will cause it to start ripping, and its not water resistant. water-repelling covers would be the best idea for outdoors covering.
I put mine on at night and when Im chillin in my house. I dont want the sun to fade my paint and it keeps the pollen off too during the day. It works pretty well if you ask me.
I put Ashlyn's blanket on everynight before bed, "Night Night Teggy!"
I use my oem car cover duting the day outside at work. Not on rainy days though, mainly just to keep the sun from fading paint/interior. The sun will break the car cover down over time but its a good investment. I just took mine down to a laundrymat with a large washer and washed it with detergent, but i hung it out to dry cause i didnt know if it would shrink.