Just calculating the costs means a lot of comparison crunching. It's a bit too challenging for me, particularly for something that is rather relative. I mean, are you talking about 'how much will this impact my gold income next turn?' Or something somehow deeper and more meaningful? Also, even under the best comparisons, things that can happen in the course of the next turn or by the end of the turn you place the city in can change that calculation dramatically.
Some things I think should be allowed to be a judgment call. This is like asking for a way to get your plate to calculate and display how many calories just ended up on it so you can know if you really want to eat that much or not. I mean, sometimes your eyes are just bigger than your stomach and you bite off more than you can chew. That's really an expected aspect to the experience of city expansion that you might end up regretting expanding too fast and have to then figure out how to deal with having done so. Then maybe next time you'll have a better idea of how to aim for a more perfect expansion rate.
Even the AI just asks itself if it has much margin for gold income loss rather than trying to calculate out exactly what the costs will be because that calculation is severely intense.
