I'd say that going into a wedding, a bridesmaid typically expects to cover her dress, possibly matching shoes, plus the cost of a bachelorette evening or wedding shower. Time wise, they might expect a few afternoons of dress shopping and fittings. That might still be a stretch, but they'd manage because they want to be part of their friend's wedding.
The problem is that in some cases, the expectations get a lot more extreme. The dress becomes a high end designer dress, and professional makeup and hair styling for the wedding are added. The bachelorette evening becomes a bachelorette spa weekend in Vegas, plus and engagement party, plus a traditional wedding shower. And then there are the requests to help make centrepieces, or favours, to help save money.And this isn't usually spelled out before the process starts - it creeps up on them bit by bit, and with a stressed out or demanding bride, they're reluctant to push back, or refuse to play along.
For men, it's generally tux rental or a suit - the first is generally cheaper than buying a new dress, the second is something they may already have - and a bachelor party.
Helping to pay for the wedding dress, though, is a new one on me. I have heard of brides demanding tanning sessions so the bridal party isn't pasty white or irregularly tanned, which is generally also regarded as too far.