Order form calculation help

Hi their,

i’m trying to create an online order page

i’m having probelms calculating totals from the select options

not sure if i’ve designed it correctly

i have 3 select options (with 4 different products in each)

a single checkbox for another single product

and a final select tag for amounts (500/1000/2500/5000)

i need to assign each select option a value then * it by the amount select

am i best using onchange/an array or ditching select tags for something easier

any help greatly appreciated

apologies, here are the select options and checkbox, i need to assign a monetary value to each option and then multiply that by the amount selected in the amount select option, sorry!

<Select name=Prod group 1 >
<option selected> None
<option value=“opt1”>Full
<option value=“opt2”>Single
<option value=“opt3”>Blank
<option value=“opt4”>Misc
</select>

<Select name=Prod group 2 >
<option selected> None
<option>Full
<option>Single
<option>Blank
<option>Misc
</select>

<Select name=prod group 3>
<option selected>None
<option>Type A
<option>Type B
<option> Type C
</select>

<INPUT type=checkbox name=special>

Select name=amount >
<option selected >0
<option value=“am1”>500
<option value=“am2”>1000
<option value=“am3”>2500
<option value=“am4”>5000
</select>

<INPUT size=15 name=total id=total value=“0”>

ditched that idea was making it too hard for myself, can anyone help with this code ??

<Select name=Product >
<optgroup label=“Product group1”>
<option value=“opt1”> Product a
<option value=“opt2”>Product b
<option value=“opt3”> Product c
<option value=“opt4”> Product d
</optgroup>
<optgroup label=“Product group2”>
<option value=“opt5”> Product a
<option value=“opt6”> Product b
</optgroup>
<optgroup label=“Product group3”>
<option value=“opt7”> Product a
<option value=“opt8”> Product b
<option value=“opt9”> Product c
</optgroup>
<optgroup label=“Product group4”>
<option value=“opt10”> product a
</optgroup>
</select>

<Select name=amount>
<option selected value=“am1”>500
<option value=“am2”>1000
<option value=“am3”>2500
<option value=“am4”>5000
</select>

<B>Total Cost:</B><INPUT size=15 name=total id=total value=“0”>