With orders of $25 or more, Amazon offers Prime members the AmazonBasics Microfiber Twin XL Sheet Set in Cream for $4.18 with free shipping. (Non-members can spend $49 or more to qualify for free shipping.) That's $10 off list and the lowest total price we could find. It includes one fitted sheet, one flat sheet, and a pillowcase.
Note that this item is offered through Amazon's "Add-on program" and can only be purchased with an order of $25 or more.