Coach passengers can sometimes upgrade to business class to Asia on American Airlines for as little as $350. Delta has that beat, offering upgrades on flights from Europe to the West Coast for as little as $299.
Delta makes upgrade offers, and if you don’t take the offer there’s a good chance that the price you’re offered will change. There may be offers right after you book your ticket. The price might drop 30 days prior to departure. It might drop in an offer presented to you during online check-in. Some Delta customers have become used to checking for their upgrade offer once per day.
Domestic upgrades are even cheaper. For years United Airlines has been known for selling these cheaply. They were ahead of the curve in being willing to take almost any amount for a first-class seat before awarding it as a complimentary upgrade on domestic flights. Their $59 upgrades are known as TODs or ‘Tens of Dollars’. The upgrades have even been marketed highlighting the number of people waiting for free upgrades you’d be jumping over. That’s common now with American, too.