How to orderby meta_value_num with dollar ($) sign

You could also use the posts_orderby filter:

function wpse155827_posts_orderby_price( $orderby ) {
    return str_replace( 'wp_postmeta.meta_value', 'substr(wp_postmeta.meta_value, 1)', $orderby );
}
add_filter( 'posts_orderby', 'wpse155827_posts_orderby_price' );
$theQuery = new WP_Query( array(
    'orderby' => 'meta_value_num',
    'meta_key' => 'price',
    'order' => 'ASC',
    'suppress_filters' => false,
) );
remove_filter( 'posts_orderby', 'wpse155827_posts_orderby_price' );