How to change the price ? (add_to_cart)
I thought of another way to do it.

Start with code that just adds the product to the cart without modifying.

function addToCart($product_id, $quantite) {
    commerce_cart_product_add_by_id($product_id, $quantite);
}


Then you can use a hook to modify the line item amount

//alter the price in cart &amp; order
function my_module_commerce_cart_line_item_refresh($line_item, $order_wrapper) {

    $price = 100; //1 dollar
    $line_item-&gt;commerce_unit_price[LANGUAGE_NONE]['0']['amount']=$price;
    //alter the base_price component
    $line_item-&gt;commerce_unit_price[LANGUAGE_NONE]['0']['data']['components']['0']['price']['amount']=$price;

}


If this doesn't work try forcing the cart refresh 
e.g.

function addToCart($product_id, $quantite) {
    global $user;
    commerce_cart_product_add_by_id($product_id, $quantite);
    $order = commerce_cart_order_load($user-&gt;uid);
    commerce_cart_order_refresh($order);
}