It is like 3 diamonds to like 18. It honestly depends on what the other person will pay. I've sold them from a range from 3 diamonds all the way to 18 Dias.
Depends what a person would pay for an untextured block that will remain untextured no matter what you do with it (Place it break it etc). I just think it's cool that its a wooden slab you have to break with a pickaxe and that it remains an untextured item when placed/broken (it looks like a normal oak slab when placed).