The black ones that wallmart sells are pretty good. Water proof and has a elastic band running the full length of the cover. Will not blow off. Very nice cover for the money. Better than some that i have spent a lot more for.
I just bought my second Dowco extra large for the Rocket. The first lasted outside on the bike for five years though lots of Rain, wind and snow. It even managed to survive a few hail storms. Order them on line though as they are cheaper than any bike shop I've seen.
As for @sonny above, I bought an inexpensive bike cover at AutoZone for my project bike ('81 XS650) and must agree that it is holding up quite well so far after being on the bike for 6 months. It doesn't have the elastic band around the bottom hem but the strap underneath works well enough for me.
The last one I got from Walmart was about $60, and was really heavy material. It's made for covering the bike even when towing. I was surprised at how well it was made.