The answer above was so close; the key to my success was $cart_item
:
function price_count_cart( $price_cart, $cart_item, $cart_item_key ) {
$product_id = $cart_item['product_id'];
$count_cart = get_field('count', $product_id);
if (!$count_cart) {
return $price_cart;
}
else {
return '<span class="custom-prc">'.$price_cart.' / <small>'.$count_cart.'</small></span>';
}
}
add_filter( 'woocommerce_cart_item_price', 'price_count_cart', 10, 3 );