Out of curiousity, I’d like to see an answer about this. I won a car in in the 90s and got two 1099s, one from Subaru USA and one from the dealer - with different amounts. I saved an ad from the dealer the weekend I took delivery advertising the vehicle and used that price (lower than either 1099) on my tax return. My rationale is the ad price is what it was available for and what I’d pay for it. Sent a copy in w/ my return, ignoring both 1099s. Didn’t get push back from IRS. Maybe they didn’t audit.

If I won airline tickets, I’d challenge any amount over what I could have purchased the trip for on the day I booked the travel.
Do not take this as advice - I ain’t no stinkin’ tax attorney.

As for 2 economy tickets, I rode back from LHR in a plane with full First, full Business, and maybe 1/3 occupied economy. UA seems to have more coach seats to give away than front cabin(s).

You don't have to be a tax attorney to follow this logic. You pay taxes on the Fair market value (FMV) or what you could reasonably buy it for on the day the prize was awarded. The prize giver provides an approximate retail value (ARV), which is usually, but not always, overinflated. It's like the Price is Right -- nobody pays Manufacturers Suggested Retail Value, but that's what you have to bid.

You'll ideally want the sponsor to change their 1099 to reflect this. Some will, some won't. Barring that, it's a good idea to fill out a form 4598 to reflect said difference. But always have documentation.

To make it actionable for this thread, if you win the sweepstakes, go to UA.com that day and document (book your ticket and get a receipt then cancel, or just take screencap of final price ex-taxes) the pre-tax values for whatever dates you are going to fly. Ask that the 1099 be reflective of that.