Backed cabs are doublets and should be sold for less than a solid cab IMHO. Vince and I do not sell our cabs by carat weight (and we don't normally cut doublets). We do weigh and measure our cabs and put that info on a little sticker on the back so folks know what it is, how much it weighs and the dimensions, but we price our cabs based on the material, how much we paid for the rough and the cut. If it is a rarer material or difficult to cut, the price goes up. All our cabs are "designer" cabs, i.e., we use the very best parts of the rough to get the most appealing stone possible. Lots of waste doing that, but that's the difference between mass produced cabs from overseas and designer cabs.
Unless a stone is too soft or the seam it came from is too thin to make a cab that can be used in jewelry safely, there is absolutely no reason to back it. An obvious exception is when a clear agate with inclusions is used with a colored backing to create a cab that is more beautiful, which is an artistic expression.
Turquoise is often backed because it is soft, but the Mohave turquoise is a man-made stone that has resin and brass or some other metal veining in it. I have cut it, and it is pretty tough stuff. There is no reason to back it normally.
Just my opinion on all this, of course.
