For me it's almost always the fabric. I can generally pull enough fabrics that work together out of my stash for almost any quilt.
Recently I pulled almost enough fat quarters plus borders and bindings for a queen size Orion Star Thangles quilt. I ended up buying three orange fat quarters because they fit into the color range better than what I had on had but that's all I bought and I really didn't have to buy it. I did buy the backing but I found one that goes with the top for 50% off.