hi
when the replacement policy for the battery for the iPhone 6s was initially announced, there was quoted the fee approximate to the one you mentioned (the exact amount per country is different but always approximate to the amount you listed).
however, after that initial announcement, within a period of weeks, the policy was amended to state that if in the past you paid for any battery replacement for the iPhone 6s, that a battery change made under this new policy would be free of charge.
for example, here is a quote from the macworld (not officially associated with apple):
quote
I recently paid full price to replace my battery. Can I get a refund?
Yes! Apple has announced that it will
refund the $50 difference to all customers who paid to have an out-of-warranty battery replacement between January 1, 2017, and Dec. 28, 2017. Apple will contact eligible customers via email between May 23, 2018 and July 27, 2018 with instructions on how to obtain the credit. if you don't get an email, you can contact Apple Support and plead your case.
end quote
so if you might want to check if in your case a fully paid for battery change can be refunded.