html for an output calculator

[shadybuck]

Working on our company's new website, I am trying to set up an online price estimator for my company's aluminum gangway product, I already used html to create pulldown tabs with widths of 3', 4', and 5', and lengths of 25' every 5' to 50', I have also created check box for if a customer wants a transition plate at the end of their gangway. I have also created a button that says calculate.

My problem is, how do I create an output box that will calculate an estimate for our potential customers?

Here is an example of what we would like it to look like
http://gangways.biz/pricing/price-residential.htm

Here is the script that I have written so far

<form action="/cgi-bin/example.cgi" method="post">
    <div style="text-align: left;">Width: <select>
    <option selected="selected">3'</option>
    <option value="4">4'</option>
    <option value="5">5'</option>
    </select>        Length:  <select>
    <option selected="selected">25'</option>
    <option value="30">30'</option>
    <option value="35">35'</option>
    <option value="40">40'</option>
    <option value="45">45'</option>
    <option value="50">50'</option>
    </select> <br />
    <br />
    Extra:<br />
    </div>
    <input name="Transtion Plate" type="checkbox" value="on" /> Transition Plate<br />
     <br />
    <input type="submit" value="Calculate" /><br />
    <input onclick="fido(this.form)" type="button" value="Calculate" />            $ <input name="Calculate" align="left" type="text" size="10" value="Length * Width" /><br />
</form>

note*: the length * width was just to see if it worked

thank you for any help