@TechyInAZ
I thought I remembered reading that the largest volume of sales of discrete GPUs was in the $150-$300 price range a while back (when AMD was promoting the first generation of Polaris cards), but unfortunately I can't find a source that provides actual numbers at the moment.
I will say, anecdotally, whenever I've seen the word "mainstream" used in the context of graphics cards by someone in the tech industry or press (which is fairly often), it is used to refer to cards with MSRP <=$300, maybe a bit higher. So in the Nvidia lineup, it'd be up to an [X]60, or occasionally an [X]70 depending on the price. So if we assume that the price range commonly referred to as mainstream really is what the mainstream gamer is buying, then anyone buying a high end card is not a mainstream gamer by definition (I'm treating "mainstream", "average", "casual", "regular" etc. gamer as being synonymous here).
Of course, it's entirely possible that convention of calling $150-$300 cards is inaccurate and doesn't actually represent what the average person is buying.