You can do this by creating a module that uses the checkout_cart_product_add_after and checkout_cart_update_items_after observers.
Then inside your observer class you can put the following function in to set the price:
public function yourAddToCartFunction($observer) {
if ($p = $observer->getQuoteItem()->getParentItem()) {
$discount_amount = $your_discount_logic;
$p->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #configs
} else {
$p = $observer->getQuoteItem();
$discount_amount = $your_discount_logic;
$p->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #simple products
}
}
You will more than likely need a different function for the cart update observer call. The one above is for the cart_add_after event. The function for the cart update is nearly identical except you have to go through the cart object to get the logic.
public function yourCartUpdateFunction($observer) {
$cart = $observer->cart;
$quote = $cart->getQuote();
foreach($quote->getAllVisibleItems() as $item) {
if ($p = $item->getParentItem()) {
$discount_amount = $your_discount_logic;
$p->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #configs
} else {
$discount_amount = $your_discount_logic;
$item->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #simple products
}
}
}
The reason you would want the second function is if the logic needs to happen again if the cart is updated. If the price is going to stick forever no matter what happens once it is in the cart, you can probably exclude the second function.
MASSIVE EDIT:
Ok this is how I would do this. I am assuming you are starting with a basic, functional module.
The first thing you want to do is setup your observers in your modules config.xml file. In this case you are going to use be using the checkout_cart_product_add_after and checkout_cart_update_items_after observers. You will set that up by adding the following to your config.xml file:
<config>
...
<global>
...
<events>
<checkout_cart_product_add_after>
<observers>
<call_this_something_unique>
<type>singleton</type>
<class>Ocaff_Custompricing_Model_Cart_Observer</class>
<method>calculateAddToCart</method>
</call_this_something_unique>
</observers>
</checkout_cart_product_add_after>
<checkout_cart_update_items_after>
<observers>
<call_this_something_unique_2>
<type>singleton</type>
<class>Ocaff_Custompricing_Model_Cart_Observer</class>
<method>calculateCartUpdate</method>
</call_this_something_unique_2>
</observers>
</checkout_cart_update_items_after>
</events>
...
</global>
...
</config>
Basically what this does is it tells Magento that when the checkout_cart_product_add_after event is triggered, run the calculateAddToCart method of the Ocaff_Custompricing_Model_Cart_Observer class and also to run the calculateCartUpdate when the checkout_cart_update_items_after event is triggered.
Ok now that you have your configuration put together you can create your model. In the case of this example the class would be located in the /app/code/local/Ocaff/Custompricing/Model/Cart/Observer.php file (however you will put the class that matches your module)
Ok now in your Observer.php file you are going to put the following code:
<?php
class Ocaff_Custompricing_Model_Cart_Observer {
public function calculateAddToCart($observer) {
if ($p = $observer->getQuoteItem()->getParentItem()) {
$discount_amount = Mage::helper('custompricing')->calculatePrice($p);
$p->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #configs
} else {
$p = $observer->getQuoteItem();
$discount_amount = Mage::helper('custompricing')->calculatePrice($p);
$p->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #simple products
}
}
public function calculateCartUpdate($observer) {
$cart = $observer->cart;
$quote = $cart->getQuote();
foreach($quote->getAllVisibleItems() as $item) {
if ($p = $item->getParentItem()) {
$discount_amount = Mage::helper('custompricing')->calculatePrice($prpoduct);
$p->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #configs
} else {
$discount_amount = Mage::helper('custompricing')->calculatePrice($p);
$item->setCustomPrice($discount_amount)->setOriginalCustomPrice($discount_amount); #simple products
}
}
}
}
Ok a few notes here. both functions take a single paramater that I have called observer (and pretty much everywhere else calls it that too). This is a single variable that holds all the information that Magento passes along to all the functions that handle the event. In the case of the calculateAddToCart function we are only grabbing the Quote Item (which is confusing since in checkout with Magento there are quite a few different objects that kind of all represent the same thing if you are looking at it from 50,000 feet). What this function does is takes the quote item and determines if the product that was added to cart is a simple or configurable product and takes that appropiate action.
The calculateCartUpdate function basically does the exact same thing, but it has to instead iterate through all the products in the cart since in Magento you update the entire cart all at once, not every line item.
In both functions you will notice that I am updating the Custom Price and the Original Custom Price field. I am not 100% sure why you have to update both (and really why ask questions if it works :))
Ok now you may or may not notice that I have done something a little less than conventional for figuring out the $discount_amount variable. I am calling a helper function to get the price that the product should be set to. I put this into a helper because you may want to access that logic elsewhere in the system and I generally prefer to put that kind of logic into a Helper (in fact I am pretty sure you are going to want to use this on the product and category pages since you are modifying the way pricing is working for some products and customers get mad when prices change up unexpectadly).
Ok now onto the helper function. Since I am not 100% sure what logic you really want in the end, we are just going to keep this function simple and have it return $2. This will make everything that goes into the cart $2 no matter what its price actually is. This is going to be where you figure out your logic for pricing.
<?php
class Ocaff_Custompricing_Helper_Data {
public function calculatePrice($product=null) {
// Your custom logic will go here. Whatever number you come up with and return will be the price that the item is set to
return 2;
}
}
For the most part this should get you about 90% of where you want to be. Try this out and post questions and I'll do my best to help you.
Another Edit: Activating Helpers in config.xml
Put this code inside your config.xml file right after your observers block detailed above. This will tell Magento the location of the helper files for your module.
<helpers>
<custompricing>
<class>Ocaff_Custompricing_Helper</class>
</custompricing>
</helpers>
