adding up numbers in a web form

Hello,

I am struggling with javascript. I have an input box and would like it when numbers are inserted it will add them together defined on a set number.

So for example a price set at £50 when 2 is entered it shows the sum of £100. The part of my form is below when I would like to implement this functionality.

<div class="controls">
                <input type="text" placeholder="How many would you like?" id="ENI_WHtest_ipCount" class="inputWide" maxlength="30">
            </div>

Code (markup):

 

could you be more specific? you have a defined price set(50) and the number you add in input is the "quantity"(2) outputting 100?

 

Fairly simple: http://jsfiddle.net/rmXbJ/