Okay, so the science is kind of a wash. And late founded cities are not going to be contributing experience units for fighting. The OP is not talking about a CV, but what about the increased cost for social policies? I figure each late city means one less SP by the end of the game. Except for CV, this is not going to mean the difference between a win or a loss, but am I probably being too conservative about this? How much culture does a city need to be generating to offset the increased SP cost?
culture cost is a harsher penalty. the mechanics are similar to the beaker cost, but the penalty is higher (i think 20% per city or something - this value changed a few times from vanilla to BNW) and a large chunk of your total culture is from sources that can't be duplicated in every city (wonders, culture from city states, culture from great works, guild specialists).
it's harder to make a similar "rule of thumb" for culture/social policy progress since some of the main culture contributors are not guaranteed (cultural CS allies, culture wonders). if the culture from your core cities isn't very impressive, it's obviously easier for new cities to get enough culture output to break even - if you focus on culture in your capital (and the first 2-3 extra cities) a lot, it's almost guaranteed taht additional cities will slow you down in this regard.
circumstances where this effect is reduced are combinations of liberty + large/huge map sizes (both factors reduce the % penalty per city - liberty on a huge map reduces the penalty to a point where it's almost negligible) and/or lots of terrain based culture (brazilwood camps, chateaux, pantheons with culture from terrain improvements, landmarks and similar effects)