Расчет текстовых полей

$('[name^=pn]').on('change', function() {
	let neg = 0
	let pos = 0

	$('[name^=pn]').each( function( i, e) {
		var quantity = e.options[e.selectedIndex].value;

		if ( quantity==='0') {

		} else if ( quantity==='1') {
			var num = Number( document.getElementById( $(this).data('target')).value);
			pos += num;

		} else {
			var num = Number(this.value);
			neg += num;

		}

	});

	document.getElementById('positive').value = pos;
	document.getElementById('negative').value = neg;

});

<script src = "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
Price 1:<br />
<input type = "text" id = "p1" name = "p[]">
<select id = "pn1" name = "pn[]" data-target = "p1">
  <option value = "0"></option>
  <option value = "1">+</option>
  <option value = "2">-</option>
</select>
<br />
Price 2:<br />
<input type = "text" id = "p2" name = "p[]">
<select id = "pn2" name = "pn[]" data-target = "p2">
  <option value = "0"></option>
  <option value = "1">+</option>
  <option value = "2">-</option>
</select>
<br />
Price 3:<br />
<input type = "text" id = "p3" name = "p[]">
<select id = "pn3" name = "pn[]" data-target = "p3">
  <option value = "0"></option>
  <option value = "1">+</option>
  <option value = "2">-</option>
</select>
<br /><br />
Total +:<br />
<input type = "text" id = "positive" name = "positive">
<br />
Total -:<br />
<input type = "text" id = "negative" name = "negative">
<br />

//declare shortcuts
const inputs = document.getElementsByTagName("input");
const selects = document.getElementsByTagName("select");
const ipr = "p";
const spr = "pn";
//loop through and add event listeners to all text boxes and dropdowns
for (var i = 0; i < inputs.length; i++) {
  inputs[i].addEventListener("input", Compute)
}
for (var i = 0; i < selects.length; i++) {
  selects[i].addEventListener("change", Compute)
}
function Compute() {
  //set total positive and negative to zero
  let pos = 0;
  let neg = 0;
  //loop through each text box-dropdown combo
  for (var i = 1; i < 4; i++) {
    //determine if sign is positive or negative and add or subtract accordingly
    switch (document.getElementById(spr + i).value) {
      case "1":
        pos = pos + Number(document.getElementById(ipr + i).value);
        break;
      case "2":
        neg = neg - Number(document.getElementById(ipr + i).value);
        break;
      default:
        break;
    }
  }
  //display each total on the page
  document.getElementById("positive").innerHTML = pos;
  document.getElementById("negative").innerHTML = neg;
}

Price 1:<br />
<input type = "number" id = "p1" name = "p[]">
<select id = "pn1" name = "pn[]">
  <option value = "0"></option>
  <option value = "1">+</option>
  <option value = "2">-</option>
</select>
<br />
Price 2:<br />
<input type = "number" id = "p2" name = "p[]">
<select id = "pn2" name = "pn[]">
  <option value = "0"></option>
  <option value = "1">+</option>
  <option value = "2">-</option>
</select>
<br />
Price 3:<br />
<input type = "number" id = "p3" name = "p[]">
<select id = "pn3" name = "pn[]">
  <option value = "0"></option>
  <option value = "1">+</option>
  <option value = "2">-</option>
</select>
<br /><br />
Total +:<br />
<p id = "positive">0</p>
Total -:<br />
<p id = "negative">0</p>