Javascript Array Sum

If you can get the values in an array, you don't have to use jQuery to sum them. You can use methods already present on the array object to do the work.

Arrays have a .reduce() method. Documentation: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array/Reduce

Array.reduce accepts a function as an argument that acts as an accumulator callback. The accumulator function accepts 4 arguments (previousValue, currentValue, index, array). You only need 2 of them to sum. Those 2 arguments are previousValue and currentValue.

var sampleArray = [1, 2, 3, 4, 5];
var sum = sampleArray.reduce(function(previousValue, currentValue){
    return currentValue + previousValue;
});

If you are faced with a compatibility issue where the target environment doesn't support ECMAScript 5 or above additions, use the prototype definition defined in the MDN article linked. (Appended below)

if (!Array.prototype.reduce) {
    Array.prototype.reduce = function reduce(accumulator){
    if (this===null || this===undefined) throw new TypeError("Object is null or undefined");
    var i = 0, l = this.length >> 0, curr;
    if(typeof accumulator !== "function") // ES5 : "If IsCallable(callbackfn) is false, throw a TypeError exception."
        throw new TypeError("First argument is not callable");
    if(arguments.length < 2) {
        if (l === 0) throw new TypeError("Array length is 0 and no second argument");
        curr = this[0];
        i = 1; // start accumulating at the second element
    }
    else
        curr = arguments[1];
    while (i < l) {
        if(i in this) curr = accumulator.call(undefined, curr, this[i], i, this);
        ++i;
    }
    return curr;
    };
}

Change your HTML to use class instead of id (id must be unique):

<td>
    <input type="text" 
        style="width: 60px; background: none repeat scroll 0% 0% rgb(255, 255, 255);" maxlength="4" 
        class="unitprice" name="unitprice[]">
</td>
<td>
    <input type="text" 
        style="width: 60px; background: none repeat scroll 0% 0% rgb(255, 255, 255);"
            maxlength="4" class="unitprice" name="unitprice[]">
</td>

Then you can total via JavaScript (using jQuery .each() function):

var totalUnitPrice = 0;

$('.unitprice').each(function(index) {
    totalUnitPrice += parseInt($(this).val()); // parse the value to an Integer (otherwise it'll be concatenated as string) or use parseFloat for decimals
  });

function getSum(){
    var ups = document.getElementsByName('unitprice[]'), sum = 0, i;
    for(i = ups.length; i--;)
        if(ups[i].value)
            sum += parseInt(ups[i].value, 10);
    return sum;
}