I don't really worry about my backing matching the top. As long as it has the same color family, I'm happy. I often piece backings and have never had a complaint from the LAs that I use. Wide backs aren't big enough for large king size quilts, so I have to piece them anyhow. 10 yards will do it.
A couple of days ago, I bought 15 yards of 3 different fabrics @ Hancock's of Paducah. 15 yards bc that's when you get their quantity discount. The first one is a light blue and green watercolor. $5.99/yard up to 15 yards, when the price dropped to $4.79/yard. The extra 5 yards cost me $11.85 and it's definitely a fabric I can use for either a smaller quilt back or in quilts.
Two other fabrics I bought were Kaufman batiks on clearance at $4.99/yard with a 15 yard price of $3.99/yard. I bought 15 yards of each. 30 yards is enough for 3 backings, so it'll be about $40 each - not bad for a large king. All will give me some scraps when I trim away excess after quilting.
I've seen people say it's too difficult to piece a back, but I don't find it troublesome. Long, straight 1/2" seams with no matching points or corners only takes a few minutes. Even if there's a pattern that I carefully match (I don't usually bother, these would really be considered more like a blender than a pattern), it isn't difficult.
It's also a good opportunity to use up fabrics, either no longer loved or odds and ends. My Carolina Christmas quilt, my first ever Bonnie Hunter quilt, has chunks of the fabrics pieced into the backing. I didn't want a bunch of Christmas fabrics in my stash, so sewed them together.