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