I've been a "car guy" all my adult life, and my daily driver for the past 21 years has been a 1962 Volvo with God-knows how many hundreds of thousands of miles on it which I bought for exactly $1K. In 2009 we refreshened a spare engine I had purchased for $400 to the tune of about $550 because the old one started blowing smoke. Last year I splurged and had a Volvo specialist go through the entire car and fix "everything," which has cost me . . . uhh . . . $2,762. My mistake. But now it's ready for the next 10 years of pretty much expense-free motoring.
I know this sounds extreme, but the lesson to be learned here is that you can get a great used car between $3K and $5K. If you choose the car not based on color and rim size and the dimensions of the LCD screen but on reliabilty, first and foremost, you should be able to drive it for many years for relatively little money. A used Honda, Toyota, or even Ecotec engine costs around $1K, so that's still cheaper than buying another car. The worst thing that can happen to you is a leaking heater core, as there's usually no cheap way to replace it, unless you have a cop car like a Crown Victoria. Rule of thumb is: the less electronics and black boxes, the better. Since you are driving a beater, I assume you don't give a sh*t about what the neighbors think of you, which helps. We still have a 2002 MINI (wife's car) sitting in the garage, with 91K miles, in superb condition, worth perhaps $3K. Not for sale, but you get the point. Since you are driving a lot, I'd suggest you look into a Prius. I hate those things, hate how they drive, but they get 54mpg and last close to 300K miles without a hiccup. You can get one for $6K with Toyota check (they pull the battery pack, clean it, check it, change the coolant, and certify it as OKAY" for $280).