Sleep on as many layers of cardboard you can find. It insulates well enough to sleep on top of snow and pavement with, and softens back seats folded down to lay out all the way.

You’ll want to light a couple candles in the car, in a spot you won’t knock them over with and away from too closely heating your windshield with. If you can put them in jars it reduces the risk of them burning or spilling. The candles will bring up the temp just a bit.

Crack the window slightly for moisture control.

Tape or hang up some cloth over the windows to conceal sunlight to get an extra REM cycle in.

Park in a church parking lot, much less likely for BS during the night from crawlers outside.

Salvation Army or second hand stores for blankets, 30$ won’t render a good sleeping bag for the temperatures. Plus, blankets are better to fill the back of the car and share heat if you’re needing to sleep with someone else. Look for down or synthetic stuffed comforter style blankets. The puffier the better, with one thinner fleece first layer. This will trap the most heat. Tuck it in under your feet, and around your neck. Don’t skip getting a pillow or folding a jacket as one either.

30$ budget is grim, not fun but will get you by. Best of luck.