These days a good-looking duvet cover easily can run you over $100. However, with a sewing machine, fabric, and a little gumption, you can have one that is custom fit to your taste and budget. On Design*Sponge, Brett Bara offers a thorough, well illustrated how-to for sewing a custom duvet cover with fabric yardage.
It goes without saying that "Cheap is relative." What one person may consider as "cheap" another might consider as expensive and still another might consider as fair-priced. So to say you want to look at "cheap daybeds" doesn't really tell anyone anything.