kittenblue, how old is your mom? If she’s from the Depression/WWII era, forget about it. Her generation deeply believes that any kind of suffering is worth pinching a few extra pennies, and no amount of logic will change that.
Complicating matter is the fact that, as JustAnotherGeek pointed out, she’s your mother. Mothers always think they know best, and we’re just kids who don’t know better. There is a way to beat her at that game, however, if she’s playing the Mom Card. And that’s to play the Rebellious Teenager Card.
You see, every Mom has a blind spot. It’s something we learn as kids, perfect as teens, and tend to forget about after we become adults. It’s what enabled you to sneak around behind her back, conceal bad grades on a report card, wear obnoxious clothing for the sole purpose of making her freak out, that sort of thing. It may take you awhile to recall what her particular blind spot is, but don’t get frustrated, eventually you’ll remember. If you need pointers…ask your son. 
Some things you might try:
(1) Change the thermostat to 72 when she’s not looking, and pretend you don’t know how it got changed. Blame the cat, the ozone layer, any wild excuse that comes to mind. (Note: this method works best with a sibling, because the two of you can conspire to blame each other.)
(2) Tell her you’ll cover the extra expense of running the A/C lower, and promise to pay her the difference when the next bill comes. The key word here is promise. When bill time comes, make excuses for not being able to pay quite yet…maybe give her ten bucks, and hope she’ll forget to ask the rest. Eventually, of course, she’ll cancel the deal and turn the thermostat back up…but by that time, summer’s over!
(3) Move into the living room, complain incessantly that it’s too hot upstairs, she just doesn’t understand, she doesn’t really love you, blah blah. Spend LOTS of time talking on the phone late at night, play loud music (Beatles, Led Zeppelin, or Metallica work best – whichever type of music her generation hates.) Oh, and when I say move in, really move in, with all your pillows and books and stuffed animals and everything. Make the living room your permanent nest. (This part is important!!!)
Whichever method you choose, be sure to make a game of it. You don’t want to awaken any emotional demons, that would be counter-productive.
Good luck…hope you find a way to stay cool!