Also keep in mind that those who make grave markers can custom make a stone to your specification. Though if you can actually contact the stone supplier instead of the grave marker seller, you can certainly save a lot of money.
Also, companies that make Kitchen Counter Tops can also make custom designs. The custom kitchen counter tops come in two types, natural stone in a variety of colors, and in man-made stone where quartz crystals (essentially granite) are glued into custom solid shapes. Again, many colors and finishes available.
I haven't priced these, but I would not expect them to cost a king's ransom. As before, the closer to the supplier and the fewer middle men that are in the mix, the lower the cost will be.
Steve/bluewizard