Silpoly is silicone impregnated polyester. It is used to make a lightweight durable waterproof fabric as the silicone strengthens the polyester fibers. It is very popular in lightweight backpacking gear. Cottage manufacturers have been using it for years, but big manufacturers have been using it recently.

Silpoly is more expensive than PU coated poly and is not quite as waterproof, though the stuff they are selling now is very good and I can't tell the difference in a tent on a rainy day. I think 190t will be quite a bit thicker than an equivalent strength silpoly.

Cottage guys who do custom work have to charge more because of the amount of labor involved. As a result of the higher base cost, they tend to use higher end fabrics because it is less of an increase as a percentage of the pieces cost. They may be willing to use whatever fabric you prefer.