Maybe you want the customer to instantly see what the order total will be if he chooses the quantity on the product page. Just copy-paste the code below at the end of the ProductDetails.html template file.
var a = '%%GLOBAL_ProductPrice%%'; a = a.replace("$","");
a = a.replace(",",""); var c = $('#qty_').val();
var ci = parseInt(c); var ai = parseInt(a);
var cis = ci * ai; var v = formatCurrency(cis);
num = num.toString().replace(/$|,/g,''); if(isNaN(num)) num = "0";
sign = (num == (num = Math.abs(num)));
num = Math.floor(num*100+0.50000000001); cents = num%100;
num = Math.floor(num/100).toString(); if(cents<10) cents = "0" + cents;
for (var i = 0; i < Math.floor((num.length-(1+i))/3); i++)
num = num.substring(0,num.length-(4i+3))+','+num.substring(num.length-(4i+3));
return (((sign)?'':'-') + '$' + num + '.' + cents);
NOTE: this will work if the quantity box is set to be a drop down menu.
Test it yourself!
Select the quantity from the drop-down and see how the price changes. (base price is $20)
Select the quantity:
Remember to always make a back up of your BigCommerce templates by downloading them before making this customization just in case something goes wrong.