JSFiddle - React, Tailwind, and code Playground

HTML

<form name="priceCalc" action="">
<div align="center">
<select name="squareOrBanner">
<option value="1">Square</option>
<option value="2">Banner</option>
</select>

<select name="number">
<option value="250">250</option>
<option value="500">500</option>
<option value="750">750</option>
<option value="1000">1000</option>
</select>
<br>
<input type="button" value="Figure out pricing!" onclick="price();"><br>
</div>
</form>

JavaScript

function price() {
    
    var sqBn = document.priceCalc.squareOrBanner;
    var Amt = document.priceCalc.number;
    
    var price = sqBn.value * Amt.value * 0.2;
    
    alert(price);
}