JSFiddle - React, Tailwind, and code Playground

HTML

<select class="select" name="discount" onchange="updateInput()">
  <option value="5" selected>5% discount</option>
   <option value="10">10% discount</option>
    <option value="20">20% discount</option>
</select>

<select class="select" name="cost" onchange="updateInput()">
  <option value="500" selected>$500</option>
   <option value="100">$100</option>
    <option value="20">$20</option>
</select>

<input type="text" name="price" value="">
    
<script>
function updateInput(){
    var discount = document.getElementsByName("discount")[0].value;
    var cost = document.getElementsByName("cost")[0].value;
    document.getElementsByName("price")[0].value = cost - (cost * (discount / 100));
}
</script>