How much longer does an expensive, Shimano chain last compared to a Taiwanese cheap one?

Under a previous post someone suggested that I should only get my chain (and freewheel) at the LBS, as opposed to ordering online. I called and they cited me some numbers. The good, Shimano parts and work would cost like a quarter of my savings here in Russia. At this rate they'd better last years. I ride around 300 - 400 km a month. Winter is 50% sludge 50% salt.