I order underwear in 6 packs online and never think twice about it. I've never found it distinctly uncomfortable or comfortable, I don't really feel it or think about it at all once I'm dressed - is that unusual? I can see it being uncomfortable if it was the wrong size, or you needed some sort of sport-specific performance underwear, but I can't think of any advantage expensive underwear would have over cheap.
I buy some sort of soft and comfortable bra maybe every couple of years to replace a worn out one. All my clothes last a long time because I don't use a dryer. My sports bras were not expensive either, maybe $20/pop? I have two in use now, one uniboob-style (which doesn't bother me, as it's not uncomfortable) and one normal-looking. I'm a 36D, so not particularly small, and those are fine for me. Maybe the key here is that it would never cross my mind to think or care about my tits needing to look a certain way in public.