Wow!
Thrift stores are good, too.
IKEA quilt covers go for 20-30 bucks, which is really good (especially for the quality -- which is high), but looks like most of them are available only in stores. Is there one near you?
Here's a Simply Shabby Chic example, it's not so cheap though (another one was similar and half the price):
Sets are usually the cover plus 1 pillowcase with a twin, and 2 pillowcases with queen + up.
As boomer says, Bed Bath and Beyond often has good stuff too, and you can sign up on their website to get a 20% off coupon in the mail.