JSFiddle - React, Tailwind, and code Playground

HTML

<form>
<input id="quantity" class="qty" type="text" placeholder="quantity" />
</form>

<!--display total cost here--> 
The Cost is: <span></span>

JavaScript

var cost = 25;
var input = document.getElementsByTagName("input")[0],
    span = document.getElementsByTagName("span")[0];

function update(value) {
    var totalcost = (cost * value);
    span.innerHTML = totalcost;
}

input.oninput =  function(){
    var val = parseFloat(input.value, 10);
    update(val);
};