Solution for me that worked (WC 2.5.5)
added 2 lines of code after:
if ( $this->get_price() === '' || empty( $prices['price'] ) ) {
add the following lines:
$price = $this->get_price();
$price = apply_filters( 'woocommerce_variable_sale_price_html', $this->get_price_html_from_to( $regular_price, $price ) . $this->get_price_suffix(), $this );
in class-wc-product-variable.php
around line 350, function get_price_html()