If you can spend a day in Tucson, you can get a Hertz rental car (in the city, not the airport). After Thursday noon, the weekend daily rate is ~40 USD. You can drop it off in Sierra Vista (no extra charge). After that, Huachuca Shuttle and Taxi in SV quoted me $35 for SV-AZT. It's not dirt cheap, but you're much more flexible and can even do some stocking along the way.