Create events.xml file in folder ‘Magemonkeys/Cartmodule/etc/frontend’ and use event ‘checkout_cart_product_add_after’. And paste the below code in it.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_cart_product_add_after">
        <observer name="addprice" instance="MagemonkeysCartmoduleObserverAddPrice" />
    </event>
</config>

And now create ‘AddPrice.php’ file in ‘Observer’ folder with the following code:

<?php
    namespace MagemonkeysCartmoduleObserver;
 
    use MagentoFrameworkEventObserverInterface;
    use MagentoFrameworkAppRequestInterface;
 
    class AddPrice implements ObserverInterface
    {
        public function execute(MagentoFrameworkEventObserver $observer) {
            $item = $observer->getEvent()->getData('quote_item');         
            $item = ( $item->getParentItem() ? $item->getParentItem() : $item );
            $price = 50; // set custom price here
            $item->setCustomPrice($price);
            $item->setOriginalCustomPrice($price);
            $item->getProduct()->setIsSuperMode(true);
        }
 
    }
?>