A dirty javascript function to calculate a new price based on a given price per number of items list.

gistfile1.txt

<script text='text/javascript'>
// function to calculate and update based on price and number of items
function updateInpPrice(val){
    if(!val || isNaN(val) || val == 0) val = 1;
    val = Math.floor(Math.ceil(val));
    var js_prices = [{"bounds":{"lowerBound":51,"upperBound":100},"price":"65.9900"},{"bounds":{"lowerBound":26,"upperBound":50},"price":"69.9900"},{"bounds":{"lowerBound":11,"upperBound":25},"price":"74.9900"},{"bounds":{"lowerBound":1,"upperBound":10},"price":"79.0000"}];
    var lenv = js_prices.length - 1;
    var iter = 0;
    var base = document.getElementById("inp-price");
    var multiplier = 1;
    var upper = 0;
    var quote = 0;
    if(isNaN(val) == true)
        base.innerHTML = "$" + (~~js_prices[iter]['price']).toFixed(2);
    else {
        while(iter <= lenv){
            if(js_prices[iter]['bounds']['upperBound'] > upper)
                upper = js_prices[iter]['bounds']['upperBound'];
            if(val >= upper + 1)
                quote = "Quote";
            if(val <= js_prices[iter]['bounds']['upperBound'] &&
               val >= js_prices[iter]['bounds']['lowerBound']){
                   multiplier = js_prices[iter]['price'];
               }
            iter += 1;
        }
        base.innerHTML = (quote ? quote : "$" + (multiplier * val).toFixed(2));
    }
    quote = 0;
    return false;
}
</script>                  

<div class="inp">
    QUANTITY: 
    <input type="text" name="qty" class="inp-qty" onkeyup="javascript:updateInpPrice(this.value)" onblur="javascript:updateInpPrice(this.value)" />
    <span class="inp-price" id="inp-price">$589.99</span>
</div>