I've gotten a couple of unexpected upgrades on UA from Econ when my ticket class did not otherwise qualify. And I've also been offered very low priced buy-ups. Once, from Biz to First on a SFO/ICN 747. This past January, I was on an Emirates A380 from DXB to BKK with a ridiculously cheap OW ticket (less than $300). Econ was oversold, and when I checked in on-line, they offered a buy-up to Biz for only $300. It was an overnight flight, and being a travel junkie on a budget who never expected to experience Emirates premium cabin, I jumped on it. The A380 was configured as 2 class cabin, with Biz in the back half of the upper deck. Econ was jammed, but Biz was only 30% occupied at best. Great experience.
Just gotta get lucky.