[Dilettante_Pro]

Как-то так?

<div class="inps">
    <label for="power"><p>Мощность</p></label>
    <input type="number" id="power" class="inp power" min="0" max="10000" step="100" value="0">
    <input id='range' class='range' type="range" min="0" max="10000" step="100" value="0" >

    <label for="cost"><p>Стоимость</p></label>
    <input type="text" id="cost" class="inp cost" >
</div>

<script>
var range = document.getElementById('range'),
     power = document.getElementById('power'),
     cost = document.getElementById('cost'),
     sum = 0;
     
document.querySelector("div.inps").addEventListener("input", function(e) {
   console.log(e.target.id);
   if (e.target.id == "range") power.value = range.value;
   if (e.target.id == "power") range.value = power.value;
   
   sum = power.value * 3.30;
    cost.value = (sum).toFixed(2);
});
</script>