I wouldn't try it if you really want to fly that day or aren't willing to pay the walk up fare. The gate agent probably doesn't have, and couldn't easily get, the authority to issue a ticket for less than the going rate.

The easiest way to discount a flight, if you don't mind risking sitting in airports is, buy a ticket for the last available flight (frequently cheaper than prime time flights) on a specific day, then show up hours earlier to fly standby. That doesn't work as well today as it used to, as many more flights are full or over-sold than was the case in the past.