I buy undies whenever I notice that the dog has hauled too many pairs out of the laundry hamper and chewed holes in the crotch lol... I'm a real cheapskate when it comes to underwear, I only like cotton briefs (I will spend anywhere up to $300 on a gorgeous bra, and I usually buy the matching panties, but then I never wear the undies because they're uncomfy!!) and I hate spending money on something that just keeps my bum covered, so I buy the 'bulk' packs from Big W or Kmart (you know, the 7 or 10 pairs for around $10?). The brands I buy have their sizing in '8-10' or '10-12' etc so I know if I buy the 10-12 they're gonna fit okay.
I used to buy Bonds undies but they're just so expensive these days, I'm one of those people who figures I'd rather fork out $10 every few months, than $20 once or twice a year hahaha... low-quality doesn't worry me, it's only underpants!

DH is a pain in the neck though, he never wore undies until recently (the zipper on his work pants broke and he was like, 'Thank God I'm at home, that would have been REALLY embarrassing if I'd been at work... can you buy me some undies next time you go shopping?'), and he wants quality, good looks and comfort (which usually equals $$$$$). He bought some Dolce & Gabbana trunks (like, those boyleg things? Tight but with legs on them instead of briefs?) and liked them, so now I buy that style, but men's underwear is sooooo expensive.