Change hidden field based on input field also multiplication... (cost calculator)

Hi guys, I am trying to create a Javascript price calculator based on the users input, plus I need it to do some math...

So the price per item is $1.00 per 1000 units.

So if the buyer enters in 1000, the calculator will change the amount hidden field to 1.00, and the cost div to 1.00 also.

HTML:

I want to buy <input type="text" class="fieldbox" style="width: 150px;" name="userinput"> items.

Your total cost is: <div id="cost">($)</div>

<input type="hidden" name="amount" value="AMOUNT BASED ON USER INPUT">

HTML:

I am very new to JS so any help would be great!

 

You can do something like the below code

I want to buy <input type="text" class="fieldbox" style="width: 150px;" name="userinput" onKeyUp="performMath(this.value)"> items.
Your total cost is: <div id="cost">($)</div>
<input type="hidden" name="amount" id="amount" value="AMOUNT BASED ON USER INPUT">

<script type="text/javascript">

function performMath(value) {
    document.getElementById('cost').innerHTML= value/1000;
    document.getElementById('amount').value = value/1000;
    
}

</script>

Code (markup):

 

Thank you! That worked perfectly.

 

If it's hidden, why are you calculating it client side?