The specific code for performing calculations with field values really depends on the field types you're using, but this page in the online help is a great place to start. Here's the "adding fields together" example from that page, slightly modified to multiply fields.

$(document).ready(function () {
    $('.sum').on('blur', 'input', sumtotal);
    function sumtotal() {
        var s = 1;
        $('.sum input').each(function () {
            s *= parseNumber($(this).val());
        });
        $('.total input').val(s);
    }
    function parseNumber(n) {
        var f = parseFloat(n); //Convert to float number.
        return isNaN(f) ? 0 : f; //treat invalid input as 0;
    }
});

The code works by adding the "sum" class to each field that will be multiplied, and the "total" class to the field where the total will go. This example will work if there's one set of fields to be multiplied. If you have several sets on your form, you'll need to differentiate them by using additional classes.