Doesn't look like they sell just the knob, so you're looking at over $200 for the whole idrive button cluster if you buy it new. You might be able to find one though from a salvage yard or ebay or something that might be used to save some money, or just go cheap and paint it or put a sticker on it or something. If it looks like crap, then you could just go buy a new one if it's bothering you. Here's a link though, as it's BMW part #65829285443 for the whole idrive control unit and realoem does not appear to show a separate part # for just the button.
http://www.ecstuning.com/Search/Site...ive/ES2590630/