Example: 50% discount for Gold Rank when buying content

add_filter( 'mycred_sell_content_post_prefs', 'mycredpro_custom_sell_content_price' );
function mycredpro_custom_sell_content_price( $sales_data ) {

    $user_id = get_current_user_id();
    $rank    = mycred_get_users_rank( $user_id );

    // Gold rank gets 50% off on price
    if ( $rank == 'Gold' )
        $sales_data['price'] = $sales_data['price'] / 2;

    return $sales_data;

}