WPMU DEV marketpress - give a discount, for example 10%, to all products without updating all products price value

code.php

<?php

add_filter( 'mp_product/get_price', function( $price, $obj ) {
	foreach( $price as $key => $val )
	{
		if( is_numeric( $price[ $key ] ) )
		{
			$price[ $key ] = $val * 0.9;
		}
	}

	return $price;
}, 99, 2 );