Как посчитать и передать значение в span?

document.querySelectorAll('.cartsum').forEach(e => {
  let p1 = e.previousElementSibling
  let p2 = p1.previousElementSibling
  e.textContent = (+p1.textContent.replace(',', '.') + (+p2.textContent.replace(',', '.'))).toFixed(1).replace('.', ',')
})

.cart1::before {
  content: ' - ';
}

.cart1::after {
  content: ' + ';
}

.cart2::after {
  content: ' = '
}

<div>Машина 1<span class="cart1">70,4</span><span class="cart2">30,7</span><span class="cartsum"></span></div>
<div>Машина 2<span class="cart1">31,1</span><span class="cart2">15,4</span><span class="cartsum"></span></div>
<div>Машина 2<span class="cart1">50</span><span class="cart2">3,8</span><span class="cartsum"></span></div>

document.querySelectorAll('div').forEach(el => {
    let cart1Val = +el.querySelector('.cart1').textContent.replace(',', '.');    
    let cart2Val = +el.querySelector('.cart2').textContent.replace(',', '.');
    el.querySelector('.cartsum').textContent = (cart1Val + cart2Val).toFixed(2);
});

.cartsum {
  border: 1px solid black;
  padding: 5px;
}

div {
  margin: 10px;
}

<div>Машина 1<span class="cart1">70,4</span><span class="cart2">30,7</span><span class="cartsum"></span></div>
<div>Машина 2<span class="cart1">31,1</span><span class="cart2">15,4</span><span class="cartsum"></span></div>
<div>Машина 2<span class="cart1">50</span><span class="cart2">3,8</span><span class="cartsum"></span></div>