Hi @fanta00
Could you please debug the returned object? You can do it by using var_dump() function like this:
<?php foreach( $posts as $p): ?>
<pre><?php var_dump($p); ?></pre>
This page should give you more idea about it: http://www.advancedcustomfields.com/resources/debug/.
You can also try to set the ID manually like this:
$product = new WC_Product(99);
Where 99 is the ID of your product which has a sale price and regular prices.
Thanks!