Purchasing Infant Fare
 

When purchasing infant and adult fare separately (booking the adult first, than later calling and booking for the infant)

which fare is used to calculate the 10% infant fare? The fare that the adult booked earlier? or the "current" fare (not last minute), which can be much higher?

Impossible to answer without details. Marketing and operating carriers? To/from?

JFK to AUH on Etihad, the adult fare was purchased last Dec with an error fare

basically the infant fare they want to charge me is more than the adult fare i paid

Some airlines will charge 10% of full fare. It is up to the airline what fare basis they use to charge the lap child.

It's 10% of the lowest applicable fare on the date of purchase - So they will look at an adult fare that allows the 90% discount. They will add taxes/fees to that. Likely around $250.

Obviously, it isn't 10% of a mistake fare.

This sounds incorrect as EY charges 20%, contrary to the norm of 10% with other carrriers.

It's a fixed percentage of the applied adult fare on day of purchase of the lap child ticket plus applicable taxes/fees. Whether the percentage is 10% or 20%, the dynamic is largely the same.

The applied adult fare is more commonly the prevailing adult fare for the route on the day of lap child ticket purchase than it is the adult fare at the time of ticketing the adult fare.