In the early '80's all the UA flights to Tucson came through Phoenix, there was a $25 OW walkup fare. It was possible to do as many as 3 round trips in a day, which I did on a few occasions to take advantage of opportunities in the new frequent flyer games.
Today, now that America West has cornered the TUS-PHX market, you have to bend over before they will sell you a ticket. Today the lowest cost to Phoenix is to buy two tickets, TUS-LAX and LAX-PHX on United or Southwest.