I don't understand why pillows and slip covers are so over-priced. It doesn't make sense when you can make a pillow cover for about $2. In case you'd like to give it a go, here's how I created a simple white slip cover for a pillow!
I bought a couple of flour sack towels from Wal-Mart, which cost about $1 each. I cut off one long side and reattached it so the towel would be more of a long rectangle than a square. I used some of the scraps to make loops that would hook around the button closures. I sewed the closures onto one short end of the rectangles. I then folded the rectangle around to form a square that was overlapped on one side and sewed both sides closed. Once I flipped it right-side-out, I marked where the loops reached and then sewed a button onto each of those locations. I stuffed a pillow form in the cover and extended the loops over the buttons to close the whole packet. It was a super quick and easy project, and it looks great thrown on a chair!