Policy on stanby at the airport has never been penalty plus difference in fare. You could always do an airport change, which is a flat fee, no fare difference involved.

There isn't really such a thing as standby anymore on discount (T/T+ tickets), but there is on lattitude/exec, but most of those tickets (except discount exec, I think) are entitled to same day airport changes, which trumps standby anyway. The only reason I can see to standby would be if those flights are full, and standby is the only way to have a shot.

Assuming you're on a T or T+ ticket, for what you want to do, you'd be looking at a same day airport change, which would be $150 or $75, depending on if you are in a T or T+ bucket. That is the standard.

Is it possible you could get free standby? Sure - It's happened before, and am sure it will happen again. But don't expect it - by the rules, its not allowed. In irregular ops, or if the weather is expected to get ugly, you've probably stand a much better chance.