As of right now, amber stones are needed for each level of the Quetzal Palace, even if that level has already been reached in another city. But it would seem that the spending of amber stones is meant to be associated with the size of one's empire, rather than a mere building level. To preserve that, I suggest one of the following:
1. If building a Quetzal Palace in another city, remove the amber portion of the cost for any levels that have already been achieved.
2. Base the max number of cities on the sum of all QP levels in all cities, not just the highest level in a single city.
In addition to making more sense (correlating amber stones to the actual size of the entire empire), this would make it easier to rearrange the importance of the cities. (For instance, if a player wants to treat a city other than their first as their main one, they can upgrade it beyond the first city and include the QP in that without spending amber for nothing.)