It is hard to say with just the informationn you supplied.

The .300 Win mag can be made to fit into a standard length action, so do not assume it is a magnum action.

Depending on the quality of the work and an eager buyer, it may be more than a $300 gun, but unless it can be attributed to some maker with a name and reputation, it is not likely a real expensive gun.

Unfortunately, it is almost impossible to sell any run of the mill custom gun for anything like the original cost to make it.

If you can find a copy of Ludwig Olsen's book on bolt action mausers, you may find enough information to identify the action.

FN in Belgiumicon made many actions both before, during, and after WWII.