Form and total priece field

[FOX]

Hi Guys,

I have a HTML form with 2 fields.
The first,package,shows the packages available,and the
second,Packageperiod,is a radio input type to let the user choose from
yearly and monthly package.
Now i would like to create another field,that show me 2 different
prices,one for the yearly option and another one for the monthly.


This is the form....could someone help me?





<td colspan="2"><p> <p><b>Package: <Select name="SignUpPlanId">

<option value="21">Prodotto1 </option>
<option value="22">Prodotto2</option>
<option value="11">Prodotto3</option>
<option value="203">Prodotto4</option>
</select>
</b>
<p><input type="radio" name="Packageperiod" value="mensile"
/>mensile
<input type="radio" name="Packageperiod"
value="annuale"checked/>annuale<b><BR></b>
<p> </td>


Thanks
Manuel