Generally cheaper finishes are due to demand e.g. Onkyo amps were £200 cheaper in silver than black about a month ago - this is clearly because black ones were far outselling silver ones so prices were dropped on silver ones to shift them. In rare cases, one kind of finish may cost a bit more than another, but if it costs £100 more just because of the finish and nothing due to the demand (e.g. from new), then the manufacturer has gone mad (IMO).