Calculate the cost of a namespace.
    Returns the price on success
    Returns None if the namespace is invalid or if the units are invalid

def price_namespace( namespace_id, block_height, units ):
    """
    Calculate the cost of a namespace.
    Returns the price on success
    Returns None if the namespace is invalid or if the units are invalid
    """
    price_table = get_epoch_namespace_prices( block_height, units )
    if price_table is None:
        return None

    if len(namespace_id) >= len(price_table) or len(namespace_id) == 0:
        return None

    return price_table[len(namespace_id)]