Magento - Let the user choose between exlude tax of include tax - magento

Question

I have a product with multiple options.
The user chooses how much he wants to pay for the product.
First I have created a dropdown box with the values €5,- €10,- etc.
The product price i have put to €0.
When I choose 10 euros, the product changes to 10 euros - that's good.
Now I want a checkbox where users can choose between Exclude Tax and Include Tax, so if I choose excluding tax the product will be inserted in the shopping cart as €10,00 witch is ok (excluding tax).
But when I choose Including tax the product needs to be inserted in the shopping cart as 10 /1.21 = €8,26 (excluding tax).
How can I make this possible?
EDIT:
I have the following code:
$event = $observer->getEvent();
$quote_item = $event->getQuoteItem();
if (is_array($quote_item->getOptions())) {
foreach ($quote_item->getOptions() as $optionValue) {
echo ???? . ' --> ' . $optionValue->getValue() . '<br />';
}
}
This will give me the values from the options.
But how do I get the real option_id? I get the option_type_id now.

Answer 1

If you look to below page
app/design/frontend/default/default/template/catalog/product/price.phtml
you will find below code
<?php if ($_weeeTaxAmount && $_weeeHelper->typeOfDisplay($_product, 0)): // including ?>
<span class="price-excluding-tax">
<span class="label"><?php echo $_taxHelper->__('Excl. Tax:') ?></span>
<span class="price" id="price-excluding-tax-<?php echo $_id ?><?php echo $this->getIdSuffix() ?>">
<?php echo $_coreHelper->currency($_price+$_weeeTaxAmount,true,false) ?>
</span>
</span>
<span class="price-including-tax">
<span class="label"><?php echo $_taxHelper->__('Incl. Tax:') ?></span>
<span class="price" id="price-including-tax-<?php echo $_id ?><?php echo $this->getIdSuffix() ?>">
<?php echo $_coreHelper->currency($_finalPriceInclTax+$_weeeTaxAmount,true,false) ?>
</span>
</span>
<?php elseif ($_weeeTaxAmount && $_weeeHelper->typeOfDisplay($_product, 1)): // incl. + weee ?>
<span class="price-excluding-tax">
<span class="label"><?php echo $_taxHelper->__('Excl. Tax:') ?></span>
<span class="price" id="price-excluding-tax-<?php echo $_id ?><?php echo $this->getIdSuffix() ?>">
<?php echo $_coreHelper->currency($_price+$_weeeTaxAmount,true,false) ?>
</span>
</span>
to implement you can refer
Changing the price in quote while adding product to cart: magento