yep- or a new cheapie square sleeping bag from the likes of tesco or argos. My underquilt started its life as a £7 argos sleeping bag. unzipped, cut on the diagonal into a rugby ball shape, covered in 99p a meter olive ripstop nylon, and channels added down the side for (quite thick) black elastic bungie.
Other cheap insulation comes from supermarket duvets, but is usually not in sheets- so you would have to quilt it.