Here's a quick D.I.Y. for hiding an ugly box spring mattress on a budget ($20-40).
When I first moved into my place I inherited a second hand bed with an ugly (and slightly ripped) box frame mattress. I had other priority pieces of furniture to spend my money on before a bed frame so I was looking for a quick, clean looking fix.
I got a basic linen drop cloth from Bunnings and cut it into a long strip (the hight of my mattress, plus about 4 inches). Then I applied some sticky back velcro to the drop cloth, and to the top of the mattress to hold it in place.
Tip: hand wash and iron your drop cloth before applying to loosen it up a little.