As I understand it with Delta, the "upgrade" price for a walk-up is absolutely ridiculous -- it is the price of a 1 way ticket in Business/First, plus the change fee from your coach ticket (without even a credit for the amt paid for your coach ticket if it is non-refundable). They have no such thing as a "$300" fee to upgrade one way somewhere.
EL AL -- different story entirely. There, they even have a partner site which lets you bid online for the price to upgrade, which uses some dynamic algorithm based on occupancy on the flight