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.

Code (text):

<div class="controls">

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

adding up numbers in a web form

See original post by deb0and