The only cash transactions I've ever engaged in on board with another passenger was back in my coach days I paid $5 to an 8 year old kid (check w/ his mom first) not to recline his seat for the during of a CLE-LAX flight.
I'd definitely give up my domestic F seat for $500.
I assume this was America West, which is pretty lousy to begin with....
My reserve price would be far lower than the amount on offer.