wpdb->get_var – count author posts, meta value

This wouldn’t be possible unless you keep a log of users that have voted the Idea. You will have to create another hidden post_meta array which will store a list of all the users who have voted the Idea. In that way you could count the number of votes an author has given. Or better yet you could create and update the user_meta ‘ideas_voted’ when a particular idea is voted by a user.

if ( idea is voted ) {
    $ideas_voted = ( get_user_meta( $curauth->ID, 'ideas_voted' ) ) ? get_user_meta( $curauth->ID, 'ideas_voted' ) + 1 : 1;
    update_user_meta( $curauth->ID, 'ideas_voted', $ideas_voted );
}