Costco had a coupon for $20 or $30 off nice, thick mattress toppers last summer (I don’t remember the final cost). My kid’s roommate had a thin topper and the difference between the thick Costco one and the thin one was noticeable. The Costco topper was more comfortable. The mattress was thin enough that regular TwinXL sheets fit it comfortably.
Go,to,the “good buy of the day” thread in the cafe. Someone there got a great deal on one at Kohls.