Honestly not sure yet. My wife was gassing up about every 10 days in her Toyota Hybrid Rav 4 (11.5 gallons) before, but that was when she was chauffeuring my son around to Boy Scouts, Aikido lessons, Marching Band practices, Robotics Club, clarinet lessons, generic school functions, and swimming league (yes, all of those things at the same time). The boy just got his license though, so he is inheriting the RAV 4 and the wife gets the new car. We still have the other son who has Boy Scouts, Kickboxing, Gymnastics, and trombone lessons, so she'll still put on some miles, but not just as much before.
Long story short, assuming about half the driving has gone away, she still would have filled up about 20 times a year for about $850 in gas (as in, previously we were spending $1700 a year). Rough estimates for electricity usage for 300 miles of travel (one tank in the RAV4), the same travel will cost us about $127 of electricity (@ the new 20 times a year). So, I'm thinking about $700 a year savings. Just is really going to depend how much less we use it now that son #1 is driving himself.
We were looking at beater cars, but couldn't find anything under 25k that didn't have at least 60,000 miles on it already. Car lots (both new and used) are just EMPTY right now. So we ended up deciding to get the new car for the wife (RAV4 was just paid off a couple of months ago, has about 50K miles). This way son #1 should have a car that will get him through college without too much worry about break-downs / maintenance.