Wow, that's kind of shocking actually! I was hopeful that they'd just charge you 10% of the "per flight" price of your pass! Which would have been very cheap!
With normal tickets you're paying (or at least I have, up until 2 years ago, when my kids reached the age of 2) 10% of the fare you've paid when booking, not necessarily Y. I have to check my records though for the charge when booking an infant *after* having upgraded the economy class ticket with a voucher to business class. I have paid both ~19€ and nearly 39€ for the same route and those 39€ sound like exactly 10% of Y, but I think it was an already upgraded flight.