I had an interesting situation for me last week for my EWR-LAX flight. (It was interesting for me but im sure a lot of others have had similar)

For a week leading up to my flight I had an opportunity to move up to F (but still be waitlisted) for 15,000 miles and approx. $250. I didn't have any miles in my account so I was resigned to stay put in coach...

When I checked in <24, I had offer to pay for instant upgrade for $279....no miles needed.... I couldn't find my credit card quick enough.... and I took offer.....into the flat bed F seat (class ZN)....

It paid to stay put in my instance.... (though I didn't have a choice)