I'm not sure where you are or exactly what kind of millstones you have, but around here smallish millstones (24") go for about $200-300 at antique stores. The large (18-24") grinding stones (round whetstones from tool sharpeners) are anywhere from $25 to $150 on craigslist.

I have an old 24" grindstone that is very uneven around the edges. Is there any way to restore it?

i have four grist mill stones for sale from the early 1900s, 48in dia.
anybody interested email me,
thank patrick