I don't know if anyone already done the maths, so i'll try to.....
(only comparing to lightbulbing; comparing the settling requires to know when the GP's were generated, and I want to keep it simple)
From what I read in the forums:
Great Scientists are worth 1500 + ( 3 * [total population] ) beakers
Other Great People are worth 1000 + ( 2 * [total population] ) beakers
So, for the first GA, for better use you must use two non-scientist GP's ( less beakers lost).
Then ( and suposing that all cities are in research (oversimplification, I know) and that we are at 100% science ( again oversimplification) for H + C = beakers), the extra H+C total on the 8 turns fo GA must be bigger than:
2*(1000 + ( 2 * [total population] )) = 2000 + 4 * [total population] or
(2000 + 4 * [total population])/8 = 250 + [total population]/2 per turn.
From here all depends of the population and of the H + C of your empire ( none of these follows a simple formula

... ), but my feeling is that Golden ages aren't (in the optic that I used) useful before the industrial era, and maybe even there....
Of course, seeing things like this is highly reductive ( there are more things in this game than beakers) and like Arvedui and Mischief said, a well timed Hammer boost can do wonders for your game. But I wanted to do a baseline, so please forgive me...
P.S Any kind of corrections are welcome
