Well it's about sales volume not price IMO.
Indeed that's where the "Stack em high sell em cheap" model comes into play, what sort of Volume of sales do Trek see for £3k+ MAMIL machines Vs £300 commuters?
Doesn't most of their turnover comes from the cheaper end of the range?
Companies like Santa Cruz don't really operate in that cost driven market, or in high volumes, and as such they can't really be called mainstream...
It's a bit like comparing Ford with Ferrari:
Yes they both make cars, both invest in motorsport, ford even make a model or two in a similar price range to some Ferraris but I doubt you'd ever call Ferrari a "mainstream" car manufacturer....
*All IMO of course...