I need some Javascript to make my form calculate a price, below is an example of my form.

I tried many variations of Javascripts but none seem to do exactly what I want, maybe it not possible, but if it is then I believe this forum is the best place to find out.

Firstly their is a dropdown box that allows a user to select how many years they wish to register a domain name, the option value show the price for each selection.

Then the user needs to select how many domain names they wish to register, the option value shows the amount of domain names for each selection.

Finally the user is asked if they require the(se) domain names to be transferred from a previous host, if no then their is no additional cost, if yes their is a transfer fee of 4.50 for each domain name, the option value show the transfer cost.

To Summarize
The first choice needs to be multiplied by the second, for instance, if a users selects a registration period of 2 years and wishes to register 5 domain names then 6.49 needs to be multiplied by 5.

Finally if the user wishes to transfer the(se) domain names then their is an additional 4.50 that needs to be added for each domain name to be transferred. (e.g. 6.49 x 5 = x 6.49 x 4.50 = y x + y=z or a x b = x a x c = y x + y=z).



Code:
<form>
	<select name="a">
	<option value="4.49">1 Year</option>
        <option value="6.49">2 Years</option>
        <option value="9.47">3 Years</option>
        <option value="12.47">4 Years</option>
        <option value="15.45">5 Years</option>
        <option value="18.44">6 Years</option>
        <option value="21.43">7 Years</option>
        <option value="24.42">8 Years</option>
        <option value="27.41">9 Years</option>
        <option value="30.40">10 Years</option>
	</select>
	<select name="b">
		<option value="1">1 domain name</option>
        <option value="2">2 domain names</option>
        <option value="3">3 domain name</option>
        <option value="4">4 domain names</option>
        <option value="5">5 domain names</option>
        <option value="6">6 domain names</option>
        <option value="7">7 domain names</option>
        <option value="8">8 domain name</option>
        <option value="9">9 domain names</option>
        <option value="10">10 domain names</option>
        <option value="11">11 domain names</option>
	</select>
	<select name="c">
		<option value="0">No</option>
		<option value="4.50">Yes</option>
	</select>
        <select name="z">
        some form of output=z goes here
        </select>
</form>