Magento: Product dynamic price change on add to cart - magento

Question

I'd like have a product that is basically a calculator, which I will build in Javascript. I want the add to cart process to grab the generated price from the page and submit it to the cart - which is as far as I have got.
I've created an observer to hook into the checkout_cart_product_add_after event and update the quote item price based on a field value in the submitted form, which works.
The problem I have is that, if you add a second or multiple versions of the item with different prices, it updates all the other versions in the cart to the same price - so you can never have multiples of the same item in the cart with different prices.
Anyone have any ideas? Here's the code in my observer:
public function modifyPrice(Varien_Event_Observer $observer) {
$customprice = $_POST["customprice"];
$item = $observer->getQuoteItem();
$item = ( $item->getParentItem() ? $item->getParentItem() : $item );
if ($customprice > 0) {
$item->setCustomPrice($customprice);
$item->setOriginalCustomPrice($customprice);
$item->getProduct()->setIsSuperMode(true);
}
}

Answer 1

I think the only way to get this working is to dynamically create a new product with a new price or you try to work with attributes which replaces the price, whereas you have to code this logic into your shopping cart as well as in your checkout.

Answer 2

Thank you to McNab, I followed the route of adding custom options to each item added to the cart to overcome the problem with it overwriting the price - which worked perfectly.
Furthermore, I decided to actually grab the options selected on the product page that the JS uses to generate the custom price, and replicate that price generation on the back end based on the selected options. Using the following instructions to add the custom options to the product on the fly, before it's passed to the cart:
Magento - Quote/order product item attribute based on user input
This makes the price calculation much more secure, the selected options are then shown in the cart and order details, and there's no price overwriting issue!
My first question on Stack Overflow and it's hell of a community, thank you all.