It has happened, though I'm not sure if it's common or not. I worked for a big hadoop user (5 years ago: 6k boxes/over 50k cores/30 pb) that aws really wanted on their platform. They offered huge discounts to get us as a marquee customer, much better than their public pricing. Unfortunately, they were still ludicrously expensive compared to O&O but we were good at running huge automated clusters with 1.5 ops.