It depends on the situation, the person who is getting married and honestly, whether I had to travel to get there. First, I always politely turn down wedding invites to those that I don't consider a close friend. In those cases, I usually just give a card and sometimes $50. When I attend a wedding, my personal rule is generally a combination of what we can afford combined with what is estimated per plate cost to couple. Anyway, we usually give about $250.
My only exception is when my younger brother got married. I knew a year in advance so I squirrelled away as much extra $$ as possible over that year. I was the best man so I paid for his bachelor party. His friends were all in grad school or had pretty low paying jobs for the most part. They're all a bunch of great guys so I didn't want them to sweat the cost and also wanted to make sure my brother had a blast. But dang those boys can DRINK! LOL!
As for a gift, I think we gave him and his wife $750. He's the only one I'd do that for.
I did attend one destination wedding and we didn't give anything but a card to the bride/groom. She was an old college friend and made it absolutely clear to us and all our other friends that they did not want a single solitary thing for a wedding gift. She just wanted us to attend. So we did. It was awesome. We all went on a four night cruise with her and her new husband.
I thought it was really really classy of her to make sure we all didn't give her any gifts. She knew how much it was going to cost us in terms of the flight and cruise and our presence meant a great deal to her.
ETA: Another friend that got married at a resort in Puerto Rico did something I thought was very cool, absolutely hilarious and was very clear that they didn't want any gifts. They registered and the only thing in their registry was 1.) a Lamborghini or 2.) the RSVP that you'd attend. Sadly, I couldn't go due to work but Joe did. To this day, she still tells me that she's waiting for her Lamborghini.