Is there anything particularly special about the stone you are looking at supplying or is it just a matter of you being able to find cheap stone?
The reason I ask is that I believe that a good part of their fee is a markup on the stone supply, so if you supply the stone you are likely to be slugged with a surcharge anyway. We looked at purchasing our engineered stone directly from the manufacturer (via the rep) but it wasn’t worth it in the end.