JSFiddle - React, Tailwind, and code Playground

JavaScript

const cars = [
{
    price: 10000,
    car: 'audi'
},
{
    price: 10000,
    car: 'audi'
},
{
    price: 10000,
    car: 'nissan'
},
{
    price: 10000,
    car: 'nissan'
},
{
    price: 20000,
    car: 'mazda'
},
{
    price: 10000,
    car: 'nissan'
},
];

var obj = cars.reduce((p, c) => {
  const key = c.car;
  p[key] = p[key] || 0;
  p[key] += c.price;
  return p;
}, {});

var out = Object.keys(obj).map(p => {
  return { price: p, car: obj[p] }
});

console.log(out)