If we stop exporting grain from the West Coast and do not export it elsewhere, there is more grain in the US. So grain prices in the US will fall.
There is a world grain price to be sure but don’t forget to include transportation costs.
Seaborne commerce is remarkably inexpensive ~ but whether or not US grain is piling up inside that doesn’t lower the world price at all and the internal price is not going to be less than the world price ~ the markets will always make sure the sellers get the highest possible price.