Hook to main query with filters based on custom fields

Have a look at the format of a meta_query in WP_Query.

$meta_query = array(
    array(
        'key' => 'price',
        'value' => array( 500, 1000 ),
        'type' => 'numeric',
        'compare' => 'BETWEEN'
    )
);

$query->set( 'meta_query', $meta_query );